agent ui: event-driven status / model / token_usage / turn_state

new LiveEvent variants on the per-agent bus —
status_changed / model_changed / token_usage_changed /
turn_state_changed — replace the per-agent web UI's
/api/state polling for the badge row.

emit sites:
- Bus::set_model → model_changed
- Bus::record_usage → token_usage_changed
- Bus::set_state → turn_state_changed
- turn::wait_for_login → status_changed("online") on creds detect
- post_login_start / post_login_cancel → status_changed("needs_login_*")

per-agent endpoints (post_set_model / post_compact / post_new_session
/ post_cancel_turn / post_login_*) now all return 200; client
drops the post-submit refetch except on login transitions, which
still need /api/state to render the OAuth form + session stream.

client adds dispatch on the four new event kinds, threads
`currentLabel` through so the composer re-enables on a live
status flip, and no longer fires refreshState() from turn_end or
postModel — the events carry the same signal faster.

closes the per-agent half of the dashboard event-channel
refactor; TODO entry dropped.

TODO.md

@@ -23,7 +23,6 @@
 
 - **Reminder delivery-error surface**: `reminder_scheduler::tick` logs failed deliveries but doesn't persist. Add `last_error TEXT, attempt_count INTEGER` columns + a banner on the dashboard row + a "retry" affordance. Needs a sqlite migration (idempotent ALTER TABLE).
 - **Per-agent reminder status / query interface**: surface pending vs. delivered counts per agent (manager + each sub-agent) as a small chip on the container row.
-- **Phase 6 follow-ups** — dashboard side is fully event-driven (Phase 6 leftovers landed); the per-agent web UI's lifecycle endpoints (`/api/{cancel,compact,model,new-session}`, `/login/*`) still 303-redirect-and-poll. Convert them to 200 + `data-no-refresh` so the per-agent page stops refetching `/api/state` on every operator click — `LiveEvent::Note` already covers cancel/compact/model/new-session, login state needs its own `NeedsLogin` / `LoggedIn` events on the per-agent bus.
 - **Tombstones + meta_inputs events**: not yet event-derived. PURG3 + meta-update still trigger a post-submit `/api/state` refetch on the dashboard. Add `TombstoneAdded`/`TombstoneRemoved` + `MetaInputsChanged` so those forms can drop their refetch too and the cold-load is the only `/api/state` fetch in normal operation.
 
 ## Security