chore(skills): sprint-start A1c — mandatory teardown, no pre-checks

The worktree cleanup step was documented as "safe to skip if the
sprint didn't use ephemeral worktrees" — which invited a pre-check.
Sprint 36 close did that pre-check from inside the repo root, where
.sprint/ doesn't exist, concluded no worktrees, and skipped teardown.
Three stale worktrees persisted until Sprint 37 planning surfaced them.

Fix: always run the script (idempotent, prints a graceful message when
empty). Document that .sprint/ is a sibling of the repo root, not a
child, so future sessions don't repeat the same ls-based pre-check.
Add a post-run verification step.

.claude/skills/sprint-start/SKILL.md

@@ -130,17 +130,34 @@ If the user raises items that should be tracked, create Q-NNN entries
 or backlog tickets on the spot. If process changes are agreed, update
 the relevant skill files or CLAUDE.md immediately — don't defer them.
 
-#### A1c. Clean up sprint worktrees
+#### A1c. Clean up sprint worktrees (MANDATORY — do not skip)
 
-Remove ephemeral worktrees for the closed sprint. Run the teardown script:
+Always run the teardown script. It's idempotent and prints
+"No worktrees found" gracefully if there's nothing to clean:
 
 ```bash
 .claude/skills/sprint-start/scripts/sprint-teardown.sh {N}
 ```
 
-This removes all worktrees under `.sprint/sprint-{N}/` and prunes git
+**Do not try to pre-check whether worktrees exist by running `ls`
-metadata. Safe to skip if the sprint didn't use ephemeral worktrees
+locally.** Sprint worktrees live at
-(e.g. legacy persistent worktree setup).
+`$(dirname <repo-root>)/.sprint/sprint-{N}/` — a *sibling* of the
+repo root, not a child. Running `ls .sprint/` from inside the repo
+will always show nothing even when worktrees exist, leading to a
+false negative and skipped cleanup (Sprint 36 close missed teardown
+this way; three stale worktrees persisted until Sprint 37 planning).
+
+The script knows the correct path via its own `SCRIPT_DIR` — trust it.
+
+Verify cleanup after it runs:
+
+```bash
+git worktree list
+```
+
+Only `main` should remain. Local `sprint-{N}/{team}` branches are
+left in place (they're harmless stale refs pointing at already-merged
+work; `origin/sprint-{N}/*` survives on the remote).
 
 #### A2. Bump the version