fix(issue-21): PR #29 review R1-R3 - untracked-safe cleanliness checks; a worktree gets its own venv

R1/R2: `git status --porcelain` honours status.showUntrackedFiles, so a
`no` setting in a submodule (or the worktree) hid untracked files from the
checks that justify worktree_remove.sh's single --force and from the review
snapshot guard (_dirty_submodules), which then approved content it never
captured. All three checks (lib_py/worktree.py, worktree_remove.sh,
runner._dirty_submodules) now pass --untracked-files=all. The source tree's
own snapshot was already safe: `git add -A` ignores that setting.

R3: worktree_prepare.sh no longer links .venv to the control root's. It
creates the worktree's own venv with the control root's interpreter and
installs requirements*.txt; a leftover link is refused (exit 1), a control
root without an interpreter is a warning (VENV=skipped); VENV= is printed.

Tests: w1 (real venv, kept on re-run), w1b (link refused / skipped), w6e
parametrised over the hidden setting for the post-stop recheck, w6e2
(hidden untracked file in the submodule and in the worktree blocks the
removal), l17 untracked-hidden / during-hidden (guard refuses before the
first review; a mid-round hidden file makes the round stale). Each new
case fails without the fix.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

.agents/skills/lib_py/loop/runner.py

@@ -712,9 +712,10 @@ class LoopRunner:
     def _dirty_submodules(self) -> Optional[List[str]]:
         """Checked-out submodules (recursively) with uncommitted or untracked
         content; None when the check itself fails. Each submodule is asked
-        directly with --ignore-submodules=none, so no submodule.*.ignore or
-        diff.ignoreSubmodules setting can hide it."""
-        script = ('s=$(git status --porcelain --ignore-submodules=none) || exit 1; '
+        directly with --ignore-submodules=none and --untracked-files=all, so no
+        submodule.*.ignore, diff.ignoreSubmodules or status.showUntrackedFiles
+        setting can hide it."""
+        script = ('s=$(git status --porcelain --untracked-files=all --ignore-submodules=none) || exit 1; '
                   '[ -z "$s" ] || printf "%s\\n" "$displaypath"')
         r = self._git(self.worktree_root, "submodule", "foreach", "--recursive", "--quiet", script)
         if r.returncode != 0: