feat(maestro): apply the kill's worktree rule when a session exits on its own

When the liveness watcher finds the tmux session gone, it now settles
the worktree through the same probe_worktree/record_retention helpers
the kill uses: pristine trees are removed, anything else is retained as
exited-with-unsaved-work before anything is torn down. The dead session
drops its worktree reference once settled, so dismissing it neither
refuses nor retains twice. If the probe or store write fails, nothing
is destroyed and the tree stays attached to the record.

WORKTREES.md: written down the settle-then-drop and nothing-destroyed-
when-unrecordable rules alongside the existing exit rule.

docs/WORKTREES.md

@@ -97,6 +97,17 @@ Nobody is waiting for an answer here, so there is nothing to refuse; the choice
 only between removing and retaining. It uses the same predicate as the kill, from
 the same place, so the two paths cannot drift apart.
 
+The session itself stays in the catalog marked dead until it is dismissed — the
+replay buffer of a crashed agent outlives it — but its worktree is settled at
+once. Once settled, the session drops its worktree reference, so dismissing it
+afterwards neither refuses (there is no guard left to trip) nor records a second
+store entry.
+
+If settling is impossible — the probe fails, or the store cannot take the entry —
+nothing is destroyed. The tree stays on disk *and* stays attached to the dead
+record, where dismissing it later still goes through `--force`'s
+retain-before-teardown path. Losing the store must never mean losing the work.
+
 ## The retained store
 
 A `state_dir`-backed record of worktrees the daemon created that no longer have a