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, 11 insertions, 1 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index b63ff3e..2cebfef 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -2,6 +2,8 @@ add_library(qtmaildir_lib STATIC
     keymap.cpp
     config.cpp
     mimeparser.cpp
+    markdownrenderer.cpp
+    messagebuilder.cpp
     requestinterceptor.cpp
     htmlbuilder.cpp
     cidschemehandler.cpp
@@ -10,8 +12,15 @@ add_library(qtmaildir_lib STATIC
     marks.cpp
     carddelegate.cpp
     notmuchworker.cpp
+    maildirname.cpp
+    draftstore.cpp
+    messagesender.cpp
+    composecontext.cpp
+    formattoolbar.cpp
     tagchip.cpp
     tagcolors.cpp
+    senddialog.cpp
+    composewindow.cpp
     savequerydialog.cpp
     tagdialog.cpp
     tagrules.cpp
@@ -36,7 +45,8 @@ target_include_directories(qtmaildir_lib
 
 target_link_libraries(qtmaildir_lib
     PUBLIC Qt6::Widgets Qt6::Svg Qt6::WebEngineWidgets PkgConfig::GMIME
-           ${NOTMUCH_LIBRARY})
+           ${NOTMUCH_LIBRARY} PkgConfig::CMARK_GFM
+           ${CMARK_GFM_EXTENSIONS_LIBRARY})
 
 # resources.qrc belongs to the executable, not to the static library. A qrc
 # compiled into a .a registers itself from a global initialiser, and the linker