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, 17 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 97a1d90..47b0df6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -34,6 +34,23 @@ message(STATUS "Found notmuch: ${NOTMUCH_LIBRARY}")
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(GMIME REQUIRED IMPORTED_TARGET gmime-3.0)
 
+# cmark-gfm renders the composer's markdown body into the HTML part.
+#
+# TWO lookups, not one, and this is the trap: only the CORE library ships a
+# pkg-config file. `libcmark-gfm-extensions` has none (verified 2026-08-20 on
+# Slackware, cmark-gfm-0.29.0.gfm.13), so it is located by hand exactly as
+# notmuch is. The extensions library is not optional here: autolink,
+# strikethrough and tasklist all live in it, and without it a bare URL in a
+# mail body is not a link.
+pkg_check_modules(CMARK_GFM REQUIRED IMPORTED_TARGET libcmark-gfm)
+find_library(CMARK_GFM_EXTENSIONS_LIBRARY NAMES cmark-gfm-extensions)
+if(NOT CMARK_GFM_EXTENSIONS_LIBRARY)
+    message(FATAL_ERROR
+        "libcmark-gfm-extensions not found. It ships with cmark-gfm but has "
+        "no pkg-config file; it provides autolink, strikethrough and tasklist.")
+endif()
+message(STATUS "Found cmark-gfm extensions: ${CMARK_GFM_EXTENSIONS_LIBRARY}")
+
 # The version lives only in the project() call above; version.h is generated
 # from it so no source file repeats the literal.
 configure_file(