fix: include worktrees/ (non-hidden) in finishing provenance check (PRI-974)

The creation skill supports both .worktrees/ and worktrees/ directories,
but the finishing skill's cleanup only checked .worktrees/. Worktrees
under the non-hidden path would be orphaned on merge or discard.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills/finishing-a-development-branch/SKILL.md

@@ -180,7 +180,7 @@ WORKTREE_PATH=$(git rev-parse --show-toplevel)
 
 **If `GIT_DIR == GIT_COMMON`:** Normal repo, no worktree to clean up. Done.
 
-**If worktree path is under `.worktrees/` or `~/.config/superpowers/worktrees/`:** Superpowers created this worktree — we own cleanup.
+**If worktree path is under `.worktrees/`, `worktrees/`, or `~/.config/superpowers/worktrees/`:** Superpowers created this worktree — we own cleanup.
 
 ```bash
 MAIN_ROOT=$(git -C "$(git rev-parse --git-common-dir)/.." rev-parse --show-toplevel)
@@ -224,7 +224,7 @@ git worktree prune  # Self-healing: clean up any stale registrations
 
 **Cleaning up harness-owned worktrees**
 - **Problem:** Removing a worktree the harness created causes phantom state
-- **Fix:** Only clean up worktrees under `.worktrees/` or `~/.config/superpowers/worktrees/`
+- **Fix:** Only clean up worktrees under `.worktrees/`, `worktrees/`, or `~/.config/superpowers/worktrees/`
 
 **No confirmation for discard**
 - **Problem:** Accidentally delete work