docs(web-ui/agent): add POST /api/effort and effort_changed to endpoint + event list

docs/web-ui/agent.md

@@ -282,6 +282,12 @@ shaped).
   `TurnStateChanged`.
 - `POST /api/model` (`model=<name>`) — switch the model for
   future turns. `Bus::set_model` emits `ModelChanged`.
+- `POST /api/effort` (`effort=<level>`) — switch the claude
+  effort level for future sessions. Validated server-side against
+  `EFFORT_LEVELS` (`medium`/`high`/`xhigh`) — unknown values are
+  rejected rather than forwarded to `claude --effort`. Persists via
+  `Bus::set_effort`, which emits `EffortChanged`. Applies on the next
+  session start (no mid-session swap).
 - `POST /api/new-session` — arm a one-shot for the next turn to
   drop `--continue`. Emits a `LiveEvent::Note`.
 - `POST /api/logout` — three-step teardown that re-uses the
@@ -314,7 +320,7 @@ shaped).
 - `GET /api/state` — cold-load snapshot (`StateSnapshot`) consumed by
   `app.js` on page load and while `status === 'needs_login_in_progress'`.
   Includes `turn_state`, `context_window_tokens`, `qualified_label`,
-  `available_models`, `links`, and other fields described inline
+  `available_models`, `available_efforts`, `links`, and other fields described inline
   throughout this document. All subsequent state updates arrive via SSE.
 - `GET /api/dashboard-state` — lean snapshot of agent-owned fields
   fetched once per running agent by the dashboard's container row to
@@ -375,6 +381,9 @@ Bus events (new vocabulary on `/events/stream`):
   alive-badge. `rate_limited` is set when the harness detects a
   429 response and cleared when the retry sleep expires.
 - `model_changed { model }` — drives the model chip.
+- `effort_changed { effort }` — drives the effort picker chip.
+  Emitted from `Bus::set_effort` on `POST /api/effort`; applies on
+  the next session start.
 - `token_usage_changed { ctx: TokenUsage, cost: TokenUsage }`
   — drives the ctx + cost badges. Emitted from
   `Bus::record_turn_usage` at turn-end; `ctx` is the last