Integration branch combining WS0-WS5 (PRs #43-#46) + the activation wiring that
flips the safe seams ON in the run-team serve path:
- WS5 (D10): inject the Sea Haven handbook conventions into the planner prompt
via context_provider (zero-arg handbook loader; fail-safe to '' when absent).
- WS2: an allowlisted Slack /new-task starts a task on this coordinator
(set_new_task_callback adapter -> start_task; AUTHZ-01 gates it upstream).
- WS1 bind_multi_invoker() is already wired in _cmd_serve.
Coordinator gains a new_task_callback param + set_new_task_callback() (resolves
the constructor chicken-and-egg of referencing the coordinator's own start_task);
default_slack_listener_factory forwards it to the SlackListener.
NOT wired (deliberately): the P3 dispatch_node / build_verify path. Activating
it correctly needs a per-task expected_run_id bound into gated_build_verify_wiring
(plumbing that does not exist yet) AND the CI trust-boundary security re-review.
It stays inert pending that work.
Tests: +5 activation-wiring tests; _FakeCoordinator stub gains
set_new_task_callback. Full agent-team suite: 1140 passed, ruff clean.
