plan: backfill v0.16 with shipped features and check off done items

The v0.16 roadmap had drifted behind the 107 commits that landed since
v0.15.0. Reconcile it with reality: record the features and important
changes that already shipped, and check off the items they resolve.

Key changes:
- Replace the abandoned #!notrim / #!verbatim editor-directive section
  with the WYSIWYG reply model that superseded it (buffer is the source
  of truth) plus the vim/emacs b4review editor plugins
- Check off "keep trailers in-place when added via reply editor",
  resolved by the WYSIWYG buffer flip
- Add cancellable network operations + lore.requesttimeout (liblore 0.8)
- Add the 'unchanged' patch-state visual cue, coverless-trailer fix, and
  sashiko/patchwork URL fixes under review improvements
- Add a development-tooling block (ci.sh, ci-matrix.sh, ruff, pyright
  strict, ty, mypy, py3.11-3.13 fixes, submodules + tool.uv.sources)
- Add a misc-enhancements block (send-me-too, find_cover_commit usebranch
  fix, pr already-fetched scoping)

Assisted-by: claude-opus-4-8
Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>