bug: the close path ends the session while it is composing its completion summary

From docs/reports/e2e-slack-test-2026-09-20-run-3.md (P2), the live validation run of the PR #402 fixes on the-loop 19.8.3.

Seen

The approval's merge carried Closes #5; GitHub closed the issue at once; the close-poller fired; close_session SIGTERM'd the session (pane status 143) and removed the checkout while the agent was writing its completion summary. The summary never reached the room or the ticket. session.autoclosed logged merged: false seconds after the delivering PR merged.

Root cause

An issue-closed closure tore the session down synchronously — harness, registry record, checkout, stamp, cleanup — whatever the session was doing. Run 2 won the race (the session posted before the poll cycle); run 3 lost it. The merged field was reason == "pr-merged", which an issue closure can never satisfy.

Fixed (issue-405)

• A closure that reaches a live session standing on the graph's terminal node (its exit not yet recorded — finish-tasks in progress) is held: session.closing, nothing torn down. The session's the-loop graph complete <id> claim ends the hold at once; routing.tmux.finishGraceSeconds (default 300, 0 = the old behaviour) ends it regardless. session.autoclosed then carries waited_seconds and finished. A pointer anywhere else, a dead pane or an unreadable graph closes immediately as before; a reopen during the hold cancels it; the poller leaves a held item alone.
• session.autoclosed merged is true when a pull request recorded on the work item merged.
• finish-tasks and the skill state the endgame order: post the summary, claim graph complete, then close the ticket if it is still open.