fix(issue-21): reviews 8f141362 N1 and 275fdc4f R4/R5 - clean refusal for a broken worktree, no init over existing files, check retained submodule repositories

- N1 (planner-reviewer-claude-01) worktree_remove.sh: when `git status` fails
  in the worktree (e.g. its directory was deleted), judge_tree returned before
  METHOD was set and the summary line died under set -u with exit 1. METHOD
  now has its default before the function; the refusal is exit 3 with
  "nothing was changed", plus a `git worktree prune` hint when the directory
  is gone. Same for --dry-run.
- R4 (planner-reviewer-codex-01) worktree_prepare.sh: a submodule path that is
  not initialised yet but already holds files made the clone fail, and the
  rollback's `deinit -f` then deleted those files. --submodules now refuses
  such a path up front (new exit 3, nothing changed), which also makes the
  rollback's premise true: after a failed attempt, everything under a
  not-yet-initialised path was created by that attempt.
- R5 (planner-reviewer-codex-01) lib_py/worktree.py: `submodule foreach`
  visits initialised checkouts only, but a deinit'd submodule keeps its
  repository - stash and unpushed commits included - under the worktree's
  modules/, which the forced removal deletes. Every repository under modules/
  (nested ones included) is now checked directly (`--git-dir`): commits on no
  remote (HEAD included) and refs/stash with its reflog. A directory that looks
  like a partial repository cannot be checked and refuses.
- Also: the rollback loops are C-style (`seq 0 -1` counts down on BSD), and
  prepare warns that --base is ignored for an existing branch.

Tests W6i (deleted worktree directory, with and without --dry-run), W6j and
W6j2 (stash / unpushed commit in a deinit'd submodule's repository) and W6k
(existing files in an uninitialised submodule path). Skill docs and the design
doc describe the new refusal and the wider repository check.

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

.agents/skills/multi-agent-mux-stop/SKILL.md

@@ -121,7 +121,7 @@ Every check runs first and changes nothing; a refusal exits 3:
 2. running sessions under it only with `--stop` (stopped normally, conversations kept);
 3. no open job of those sessions - never cancelled automatically, they may belong to a running loop (the delegate job of a session `--stop` stops is cancelled by stop itself);
 4. a clean tree, submodules included (`status --porcelain --ignore-submodules=none`);
-5. inside submodules: no modification, no commit that is on no remote (`HEAD` included - a submodule sits on a detached HEAD) and no stash (`refs/stash` is reachable from no branch and leaves the status clean).
+5. inside submodules: no modification, no commit that is on no remote (`HEAD` included - a submodule sits on a detached HEAD) and no stash (`refs/stash` is reachable from no branch and leaves the status clean) - checked in every repository under the worktree's `modules/`, including deinitialised submodules (their repository stays there) and nested ones; a repository that cannot be checked refuses.
 
 Then it stops the sessions, re-checks the jobs, runs checks 4 and 5 **again** (a session that is being stopped can still write, commit or stash - any change refuses with exit 3, sessions already stopped) and removes the worktree (the branch is kept): plain, or - when the worktree holds submodule state, which git refuses to remove otherwise - a **single `--force` justified by checks 4 and 5**. Do not run `git worktree remove --force` by hand on such a worktree: it deletes the worktree's own submodule repositories, unpushed commits included.