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/worktree.py

@@ -155,7 +155,10 @@ def submodule_findings(wt: str) -> Dict[str, object]:
     after `submodule update`, so its commits may be on no branch, and a commit
     kept only by a tag (review R6) or a note is lost with the repository just
     the same. Stashes live in refs/stash and its reflog; the stash check owns
-    them."""
+    them. --untracked-files=all: `status.showUntrackedFiles=no` in a
+    submodule would otherwise hide its untracked files from the check (review
+    R1), and display settings must not decide what a forced removal may
+    delete."""
     gd = _git(wt, "rev-parse", "--absolute-git-dir")
     if gd.returncode != 0 or not gd.stdout.strip():
         return {"trace": False, "dirty": "", "unpushed": "", "stashes": "",
@@ -164,7 +167,7 @@ def submodule_findings(wt: str) -> Dict[str, object]:
                               "dirty": "", "unpushed": "", "stashes": "", "error": ""}
     if not res["trace"]:
         return res
-    checks = (("dirty", "git status --porcelain"),
+    checks = (("dirty", "git status --porcelain --untracked-files=all"),
               ("unpushed", "git log --oneline " + " ".join(UNPUSHED_REVS)),
               ("stashes", "git stash list"))
     errors = []