Tasks: closing a work item ends its harness session

Phase 3 of 3 (requirements → design → tasks). DAG — a task starts once its dependencies are checked.

• [x] T1 — provider contract. poller/base.py: Closure dataclass and PollProvider.owns / closure / closure_event with opt-in defaults. (AC1.7) — depends on nothing.
• [x] T2 — GitHub closure. poller/github.py: GhClient.fetch_item_state (gh api repos/{o}/{r}/issues/{n}, merged via pull_request.merged_at) and the provider's owns/closure/closure_event. (AC1.2, AC1.6, AC1.7) — depends on T1.
• [x] T3 — poller reconciliation. poller/poller.py: open_refs from the successful listing, _reconcile_closures, PollSummary.closures, PollState.forget. (AC1.1–AC1.6) — depends on T1.
• [x] T4 — close is close. webhook/dispatcher.py: _is_close_event / _close_reason replacing _is_pr_close, reason on session.autoclosed; webhook/router.py: lifecycle exemption widened to issues/closed. (AC2.1–AC2.4) — depends on nothing.
• [x] T5 — terminate the harness. runner.py: TmuxRunner.live_pane_pids + terminate_harness(session, grace, timeout, sleeper, killer); dispatcher._close_tmux calling it when retaining. (AC3.1–AC3.7) — depends on T4 (same close path) + T6 (config).
• [x] T6 — config mirror. TmuxConfig.kill_harness_on_close / harness_kill_grace_seconds from killHarnessOnClose / harnessKillGraceSeconds, hot-reloading with RoutingConfig. (AC5.1) — depends on nothing.
• [x] T7 — CLI parity. sessions close terminates the harness on the retain path; sessions attach forces -r for a closed session. (AC3.1, AC4.1, AC4.2) — depends on T5.
• [x] T8 — observability. poll.closure_detected and session.harness_terminated registered in eventlog.EVENT_TYPES; reason documented on session.autoclosed; cycle log/poll.cycle carry closures. (AC5.2) — depends on T3, T4, T5.
• [x] T9 — config schema + examples. killHarnessOnClose / harnessKillGraceSeconds under routing.tmux in .the-loop/cli-config.schema.json, the dogfood .the-loop/cli-config.yaml and the packaged skills/the-loop/templates/cli-config.yaml; make validate green. (AC5.1) — depends on T6.
• [x] T10 — unit tests. Provider closure mapping + owns, PollState.forget, TmuxConfig parsing, _is_close_event/_close_reason, terminate_harness (SIGTERM happy path, SIGKILL escalation, dead pane, missing session, ProcessLookupError). — depends on T1–T7.
• [x] T11 — integration tests. Poll-cycle closure scenarios (issue closed, PR merged, listing error, closure error, still-open item, reopen re-spawns), the webhook issues/closed scenario, and stub-tmux close scenarios (terminate + retain, opt-out, kill-tmux). — depends on T3, T4, T5.
• [x] T12 — docs. cli/README.md config rows + prose; docs/capabilities/interactive-sessions.md and docs/capabilities/webhook-triggers.md behaviour bullets + history rows; the webhook event prompt template's parenthetical. (AC5.3) — depends on T1–T9.
• [x] T14 — reviewer request (PR #96): the event filter can see a close.commands/gh_webhook.py: DEFAULT_EVENTS + resolve_events (omitted/empty → the routable set) and warn_on_missing_lifecycle_events, wired into both the initial build and the hot-reload apply; schema default + description, the packaged template comment, cli/README.md row, and four unit tests. (AC2.5, AC2.6) — depends on T4.
• [x] T13 — gates. make check fully green; evidence in execution-log.md; PR + reviewer briefing. — depends on T1–T12.