Merge branch 'compose-and-send': composing and sending mail

Item 123, built over 2026-08-20 to 2026-08-23 in thirteen tasks against
docs/superpowers/specs/2026-08-20-compose-and-send-design.md.

The application writes mail now. A composer window per message, markdown as
the body, drafts autosaving into the account's Maildir, and sending through a
per-account command on stdin rather than any network protocol of this
program's own. A countdown with an Undo stands between pressing Send and the
command running.

Two things came in alongside it. The notmuch auto-tagging hooks moved here
from the retiring `mailctl` project and learned that mail this application
files itself never arrived, so sent mail and drafts stop appearing in the
inbox. And the v1/v2 language is retired: semver on the user-visible surface
is the rule, and those labels described a split that composing made obsolete.

Hand tested against a fake send command rather than a real one, deliberately:
New, Reply and Forward all produce correct messages, a forwarded attachment
survives intact, and the sent copy is filed. That testing found the two
defects fixed on this branch, and both were invisible to the suite: a composer
orphaned by quitting the main window, and every sent message tagged `inbox`.

Twenty-two defects were found in the plan document's own draft code while
building it, which is why CLAUDE.md says to treat every code block in a plan
as a draft.

1 files changed, 20 insertions, 0 deletions

diff --git a/src/notmuchworker.h b/src/notmuchworker.h
index 9932e59..8ed878f 100644
--- a/src/notmuchworker.h
+++ b/src/notmuchworker.h
@@ -223,6 +223,20 @@ public slots:
     /// source of truth the design refuses.
     void requestFolders();
 
+    /// The Maildir root, for whatever has to compose a path under it.
+    ///
+    /// This class owns the only database handle, and the root is a property of
+    /// the DATABASE rather than of config: notmuch can split the index from
+    /// the mail with `mail_root` and `path` as separate keys, so there is no
+    /// config key the UI could read instead. Item 124 records what the wrong
+    /// accessor costs. `notmuch_database_get_path()` returns the INDEX
+    /// directory under that layout, and a destination composed from it writes
+    /// into the Xapian tree.
+    ///
+    /// Requested at startup beside requestAllTags(), and answered once. The
+    /// root does not change while the application runs.
+    void requestMailRoot();
+
 signals:
     void threadsReady(const QVector<ThreadSummary> &threads, quint64 generation);
     void queryFinished(int totalThreads, quint64 generation);
@@ -288,6 +302,12 @@ signals:
     /// asks once when its dialog opens.
     void foldersReady(const QStringList &folders);
 
+    /// The Maildir root, absolute. No generation: it is a property of the
+    /// database rather than of any query, so a late answer is still the right
+    /// one. Empty when the database could not be opened, which a consumer must
+    /// treat as "cannot compose a path yet" rather than as the root being "".
+    void mailRootReady(const QString &mailRoot);
+
     void errorOccurred(const QString &message);
 
 private: