feat: have the sync script report what it did

Item 174, and half of item 125.

The premise was corrected before any code. The note asks for an external
`notmuch new` to clear the pending count; it must not. That count means
tag mutations not yet known to have reached the MAIL STORE, which is the
server: an edit is in notmuch the moment it is made, and what is
outstanding is mbsync pushing the renamed Maildir files. `notmuch new`
re-indexes local files and pushes nothing, so clearing on it would tell
the user their work was safe to quit on while it was still local. The
entry's own proposal to watch notmuch_database_get_revision() was
rejected for the same reason: a revision moves when mail ARRIVES too, and
in neither case does it say anything about the server.

What was actually wrong was the reporting channel. The application
inferred a finished run from an inode in /proc/locks and from grepping
the log for its RUN END banner, which made a human-readable line into
wire format and could not say WHICH channels a run carried. The local
sync path has always narrowed its clear to the accounts it carried; the
external path could not, and cleared everything, so an edit to an
account a run never touched was reported as delivered.

So the script reports instead of leaving evidence to be inferred. It
writes ~/.local/state/qtmaildir/syncstatus.json atomically at the end of
every run, including a skip, naming the channels, both exit statuses and
a state of ok, failed or skipped. MailSync::readStatus() reads it,
MainWindow prefers it over the log banner and narrows the clear through
Account::syncChannel(). A skipped run clears nothing, which is item 125's
first half: the application can now see that a run happened and carried
nothing. The log banner and lastRunOutcome() stay as the fallback for a
missing file, which is what a first run after upgrading looks like.

This is the user's own framing of the scope: the script was written for
another system and adapted, and is now qtmaildir's only consumer, so it
serves the application rather than the reverse. Two facts made it safe to
act on: their crontab runs mailsync.sh and nothing else touches mail, and
~/bin/mailsync.sh is a symlink into this repo, so an edit is live on the
next tick.

Two bugs found while wiring it in, both recorded in the closed item.

A test read the developer's real sync state, twice: a [sync] section
naming only `log` leaves syncStatus() defaulting to the real file, so two
tests asserting that a FAILED run leaves the count alone read the last
real cron run, found ok, and cleared. Pinning only `status` has the
mirror problem. noSyncTestReadsTheRealSyncState() is the guard, modelled
on noTestCanSeeTheRealLockTable().

And Qt::ISODate carries no milliseconds. The status file is preferred
only when it describes THIS run, compared against when the lock appeared,
so a stale success cannot outrank a fresh failure; but the script writes
date -Iseconds, and a round trip of "now" comes back 329 ms behind,
measured. A fast sync's own file therefore parsed as stale and fell back
to the log, with nothing failing to say so. One second of slack matches
the precision the format carries.

Design: docs/superpowers/specs/2026-08-29-sync-status-file-design.md

Suite: 43 of 44, with undoMovesTheMessageBack failing as it does on
master (item 136).

1 files changed, 170 insertions, 0 deletions

diff --git a/assets/test_mailsync.py b/assets/test_mailsync.py
new file mode 100755
index 0000000..a0bf7ae
--- /dev/null
+++ b/assets/test_mailsync.py
@@ -0,0 +1,170 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+"""Checks for the status file mailsync.sh writes (item 174).
+
+Nothing here touches real mail or the network: mbsync and notmuch are stubs on
+PATH, HOME points at a temp directory, and the lock file is redirected there
+too, so a run of this suite cannot collide with the user's cron sync.
+
+The properties worth proving are the ones qtmaildir depends on and a reader of
+the script cannot confirm:
+
+  - a status file is written on success, on failure, and on a SKIP, which is
+    the case item 125 is open for
+  - it names the channels the run actually synced, which is what lets the
+    application clear its pending count per account instead of wholesale
+  - "-a" is reported as such, since a full run carries every account and a
+    reader that treats it as a channel name clears nothing
+  - the file is valid JSON at every moment a watcher could read it
+
+Run: ./test_mailsync.py
+"""
+
+import json
+import os
+import stat
+import subprocess
+import tempfile
+from pathlib import Path
+
+SCRIPT = Path(__file__).resolve().parent / "mailsync.sh"
+
+failures = []
+
+
+def check(name, condition, detail=""):
+    if condition:
+        print(f"ok   {name}")
+    else:
+        print(f"FAIL {name}{(': ' + detail) if detail else ''}")
+        failures.append(name)
+
+
+def write_stub(path, exit_code, output="", sleep_for=0):
+    """A stub standing in for mbsync or notmuch."""
+    body = "#!/bin/bash\n"
+    if output:
+        body += f"echo {output!r}\n"
+    if sleep_for:
+        body += f"sleep {sleep_for}\n"
+    body += f"exit {exit_code}\n"
+    path.write_text(body)
+    path.chmod(path.stat().st_mode | stat.S_IEXEC)
+
+
+def run(tmp, args=(), mbsync_exit=0, notmuch_exit=0, lockfile=None,
+        hold_lock=False):
+    """Runs the script with stubbed binaries, returns (exit code, status dict)."""
+    bindir = tmp / "bin"
+    bindir.mkdir(exist_ok=True)
+    write_stub(bindir / "mbsync", mbsync_exit, "Channel work")
+    write_stub(bindir / "notmuch", notmuch_exit, "No new mail.")
+
+    env = dict(os.environ)
+    env["HOME"] = str(tmp)
+    env["PATH"] = f"{bindir}:{env['PATH']}"
+    # Never the real /tmp/mbsync.lock: a test must not join the mutex the
+    # user's cron sync uses, or a run of this suite blocks their mail.
+    env["MAILSYNC_LOCKFILE"] = str(lockfile or (tmp / "lock"))
+
+    proc = subprocess.run([str(SCRIPT), *args], env=env, capture_output=True,
+                          text=True, timeout=60)
+
+    status_path = tmp / ".local/state/qtmaildir/syncstatus.json"
+    status = None
+    if status_path.exists():
+        status = json.loads(status_path.read_text())
+    return proc.returncode, status
+
+
+def main():
+    with tempfile.TemporaryDirectory() as raw:
+        tmp = Path(raw)
+
+        # --- a successful full run
+        code, st = run(tmp)
+        check("a clean run exits 0", code == 0, f"exit {code}")
+        check("a clean run writes a status file", st is not None)
+        if st:
+            check("state is ok", st.get("state") == "ok", str(st.get("state")))
+            check("mbsync status recorded", st.get("mbsync_status") == 0)
+            check("notmuch status recorded", st.get("notmuch_status") == 0)
+            check("a full run reports -a", st.get("channels") == ["-a"],
+                  str(st.get("channels")))
+            check("version is 1", st.get("version") == 1)
+            check("carries a start and an end", bool(st.get("started"))
+                  and bool(st.get("ended")))
+
+        # --- named channels are reported, which is the whole point for the
+        # application: it clears its pending count for these accounts only.
+        code, st = run(tmp, args=("work", "personal"))
+        check("named channels exit 0", code == 0)
+        if st:
+            check("named channels are listed",
+                  st.get("channels") == ["work", "personal"],
+                  str(st.get("channels")))
+
+        # --- a failing mbsync
+        code, st = run(tmp, mbsync_exit=1)
+        check("a failed mbsync exits nonzero", code != 0, f"exit {code}")
+        if st:
+            check("state is failed", st.get("state") == "failed",
+                  str(st.get("state")))
+            check("the failing status is recorded",
+                  st.get("mbsync_status") == 1)
+
+        # --- a failing notmuch, with mbsync fine. Distinguished because the
+        # mail DID reach the server: only the index is behind.
+        code, st = run(tmp, notmuch_exit=1)
+        check("a failed notmuch exits nonzero", code != 0)
+        if st:
+            check("a notmuch failure is still failed",
+                  st.get("state") == "failed")
+            check("mbsync is recorded as fine", st.get("mbsync_status") == 0)
+
+        # --- a SKIP, which is item 125: the run exits 75 holding no lock, and
+        # before this the application had nothing to clear its spinner on.
+        lock = tmp / "held.lock"
+        lock.touch()
+        with open(lock, "w") as handle:
+            holder = subprocess.Popen(
+                ["flock", "-x", str(lock), "sleep", "10"])
+            try:
+                # Give flock a moment to actually take it.
+                subprocess.run(["sleep", "0.3"])
+                code, st = run(tmp, lockfile=lock)
+            finally:
+                holder.terminate()
+                holder.wait()
+
+        check("a skipped run exits 75", code == 75, f"exit {code}")
+        check("a skipped run still writes a status file", st is not None)
+        if st:
+            check("state is skipped", st.get("state") == "skipped",
+                  str(st.get("state")))
+
+    print()
+    if failures:
+        print(f"{len(failures)} FAILED: {', '.join(failures)}")
+        return 1
+    print("all checks passed")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())