Tasks: retained tmux sessions + session announcement comments

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

• [x] T1 — tmux remain-on-exit + pane liveness. runner.py: TmuxResult.output; TmuxRunner(remain_on_exit=True); best-effort set-option -w remain-on-exit on after spawn; has_live_session() via list-panes -F "#{pane_dead}"; deliver() guards on liveness. (AC2.1–2.3) — depends on nothing.
• [x] T2 — AnnounceConfig + announcement_body. New cli/the_loop/announce.py: config mirror with from_mapping, pure body builder using only registry-derived fields. (AC3.1, AC3.2, AC3.5, AC3.6) — depends on nothing.
• [x] T3 — SessionAnnouncer. No-op ladder (disabled, non-tmux, non-github, malformed owner/repo, missing gh warn-once), gh api POST to the issue comments endpoint, session.announced / session.announce_failed, never raises. (AC3.1–3.5, AC4.2, AC4.3) — depends on T2.
• [x] T4 — Dispatcher retention. TmuxConfig in RoutingConfig; runner built with remain_on_exit; PR-close keeps the tmux session by default, logging the attach command and emitting session.retained; kills when keepSessionOnClose: false. (AC1.1, AC1.2, AC1.4, AC1.6, AC4.1) — depends on T1.
• [x] T5 — Dispatcher announcement. Optional announcer ctor param (override survives reload); _spawn_tmux announces after registration without affecting its return value; _respawn_tmux deliberately does not (first spawn only). (AC3.1–3.4) — depends on T3, T4.
• [x] T6 — sessions close / sessions attach. close honours keepSessionOnClose with --keep-tmux / --kill-tmux overrides; attach falls back to a closed work item's retained session (with a note) and stops refusing dead panes. (AC1.3, AC2.4, AC2.5) — depends on T1, T4.
• [x] T7 — Event-log types. Register session.retained, session.announced, session.announce_failed in eventlog.EVENT_TYPES. (AC4) — depends on nothing.
• [x] T8 — Config schema + example. routing.tmux and routing.announce in .the-loop/cli-config.schema.json (defaults documented) and the dogfood blocks in .the-loop/cli-config.yaml (+ the packaged template if it carries the same blocks); make validate green. (AC1.2, AC3.5) — depends on T1, T2.
• [x] T9 — Unit tests. test_announce.py plus the test_tmux_runner.py additions per the design's testing strategy. — depends on T1–T3.
• [x] T10 — Integration + CLI tests. Retention / kill-on-close, respawn through a dead retained pane, announce-on-spawn, no re-announce on respawn, no-announce for process mode, announcer failure is inert; sessions close --keep/--kill-tmux and sessions attach to a closed-but-retained session. — depends on T5, T6.
• [x] T11 — Docs. cli/README.md config tables + sessions command notes; docs/capabilities/interactive-sessions.md behaviour bullets and history row. — depends on T5, T6.
• [x] T12 — Gates. make check (ruff, markdownlint, format, pyright, validate, pytest) fully green; evidence in execution-log.md. — depends on T1–T11.