fix(issue-21): review ec50fbe7 R6 - the unpushed check covers HEAD and every local ref (tags included, stash excluded) in both submodule checks

Both unpushed checks in lib_py/worktree.py - `submodule foreach` over the
initialised checkouts and the direct `--git-dir` walk over the repositories
under the worktree's modules/ - used `HEAD --branches --not --remotes`. A
commit made on a submodule's detached HEAD and kept only by a tag is reachable
from neither, the top-level status stays clean, and worktree_remove.sh then
deleted the repository with its single --force (exit 0).

Both checks now share one rev list, UNPUSHED_REVS = HEAD --exclude=refs/stash
--all --not --remotes: HEAD and every local ref (branches, tags, notes, ...)
minus remote-tracking refs. refs/stash stays out of it because the stash check
(stash list / reflog) owns it, so a stash is reported once, as 'stashes'. As
intended, a local-only note is now protected too. The stash, error, partial
repository and dirty checks are unchanged.

Tests W6l (tag-only commit; initialised submodule and deinit'd repository) and
W6m (annotated tag). The helper header, the stop SKILL.md and the design doc
state the wider rule.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017R3Eg7MEpSWnm1oB9ZPP6G

.agents/skills/multi-agent-mux-stop/scripts/worktree_remove.sh

@@ -10,8 +10,8 @@
 # session under it unless --stop; (3) no open job of those sessions (the
 # delegate job of a session --stop will stop is cancelled by stop itself);
 # (4) clean tree, submodules included; (5) submodules: nothing modified and no
-# commit that is on no remote (HEAD included - a submodule is on a detached
-# HEAD) and no stash - in every repository under the worktree's modules/,
+# commit reachable from HEAD or any local ref (tags included - a submodule is
+# on a detached HEAD) that is on no remote, and no stash - in every repository under the worktree's modules/,
 # initialised or not. (4) and (5) run again right before the removal, after
 # --stop: a session that is being stopped can still write, commit or stash.
 # Removal is plain, or - when the worktree holds submodule state, which