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

@@ -84,11 +84,13 @@ fi
 
 # (4) clean, submodules included, and (5) submodules. A function: it runs now,
 # before any change, and again right before the removal (review R1). It sets
-# METHOD from what it saw and returns 1 on any finding.
+# METHOD from what it saw and returns 1 on any finding. --untracked-files=all:
+# a status.showUntrackedFiles=no setting (worktree or submodule) must not hide
+# files from a check that decides a forced removal (review R1).
 METHOD="plain"  # set before any early return below: the summary line reads it under set -u
 judge_tree() {
   local found=0 dirty sub trace key val
-  if ! dirty="$(git -C "$WT" status --porcelain --ignore-submodules=none 2>&1)"; then
+  if ! dirty="$(git -C "$WT" status --porcelain --untracked-files=all --ignore-submodules=none 2>&1)"; then
     refuse "git status failed in the worktree: $dirty"
     if [ ! -d "$WT" ]; then
       echo "  the directory no longer exists; if that is intended, run: git -C '$MAIN' worktree prune" >&2