One-shot startup task that GCs orphaned ex_athena git worktrees.

When a parent process crashes mid-run, its subagent's worktree may be left behind on disk. Running git worktree prune reclaims any registered-but-deleted worktrees; we additionally remove our own cache directory entries that are older than 7 days and don't have matching prune entries.

Started under the application supervisor; runs once at boot then exits. Logs are info-level — failures are best-effort.