agents: remove the hive-level /effort control too

mara on #5034: "effort shares /models fate". The per-agent web UI no
longer sets effort: the `/effort` slash command, the effort picker,
`POST /api/effort`, `Bus::set_effort`, the `hyperhive-effort` writer,
`EFFORT_LEVELS`/`is_valid_effort` and the ACP effort-list payload are
gone, and `lib/modelEffort.ts` with them. The effort chip is
display-only and shows its source (`effort_source` in `/api/state`);
on ACP it stays hidden while the session offers no effort level.

The read side is unchanged: with no swarm value an agent runs an
existing `hyperhive-effort` file, else its agent.nix effort, else
medium. Existing files are not touched.

Refs #5034

docs/web-ui/agent.md

@@ -47,21 +47,20 @@ at a given viewport width. Two columns:
     `◌ needs login` / `◌ logging in` / `○ offline` / `… connecting`.
   - **State badge**: `💤 idle` / `🧠 thinking` / `📦 compacting` /
     `○ offline` / `… booting` + age suffix.
-  - **Model badge** (`model · <name>`): display only. Its tooltip says where
-    the model comes from (`state.model_source`: set on the swarm, agent.nix,
-    an existing `hyperhive-model` file, or the built-in default) and what the
-    last turn resolved it to. The model is set on the swarm: the swarm UI's
-    agents page, or `swarmctl agent inference set`
+  - **Model and effort badges** (`model · <name>`, `effort · <level>`):
+    display only. Each tooltip says where the value comes from
+    (`state.model_source` / `state.effort_source`: set on the swarm,
+    agent.nix, an existing `hyperhive-model` / `hyperhive-effort` file, or
+    the built-in default); the model's also says what the last turn
+    resolved it to. Both are set on the swarm: the swarm UI's agents page,
+    or `swarmctl agent inference set`
     ([`inference.md`](../swarm/inference.md)).
-  - **Effort badge** (`effort · <level> ▾`): a real picker — selecting it
-    opens a `Dropdown` of `state.available_efforts` and POSTs `/api/effort`
-    immediately. Shown when `state.available_efforts` is non-empty.
-  - On an ACP agent the model badge shows the session's own model when it
-    doesn't offer the requested one, and the effort picker lists the levels
-    its session offers (its `thought_level` config option), empty until the
-    first turn has attached a session. The effort picker is hidden when the
-    model offers no effort levels, and while a newly picked model waits for
-    the next turn, since the levels come with the model.
+  - On an ACP agent each badge shows the session's own value when it
+    doesn't offer the requested one. The effort badge is hidden
+    (`state.effort` is `null`) until the first turn has attached a
+    session, when the model offers no effort levels, and while a newly
+    set model waits for the next turn, since the levels come with the
+    model.
   - **Ctx / cost badges**: `ctx · 142k` (last inference's prompt size,
     tooltip shows % of context window) / `cost · 1.3M` (cumulative
     tokens billed across every inference in the last turn).
@@ -85,8 +84,8 @@ at a given viewport width. Two columns:
     (`GET /api/state`'s `links` field) also feeds
     `DashboardState.links` for the dashboard card's icon strip —
     `agent_links()` in hive-agent is the single source of truth for
-    both. The dashboard link lives in this 🔗 popover; the effort picker
-    is a real badge above; and `/new-session` / `/logout` are
+    both. The dashboard link lives in this 🔗 popover, and
+    `/new-session` / `/logout` are
     typed slash commands with their own type-twice confirm (see
     below).
   - `/cancel` (slash command, below) is the only cancel-turn path; the
@@ -237,10 +236,6 @@ Slash commands today:
   runs the `/compact` at the next turn boundary (end of the in-flight
   turn, or `turn::run_pending_compact` when idle); output streams into
   the live panel. Works mid-turn, not only when idle.
-- `/effort <level>` — `POST /api/effort` setting the claude effort
-  level (`low` / `medium` / `high` / `xhigh` / `max`). Takes effect on the next
-  turn. `StatusChips`' effort badge calls the same endpoint; both stay
-  in sync via `StateSnapshot.effort`.
 - `/new-session` — `POST /api/new-session` (confirms first).
   Arms a one-shot on the Bus; next turn runs without
   `--continue`, dropping the resume session entirely.
@@ -275,15 +270,6 @@ shaped).
   turn — only the stdin payload differs). Flips state to
   `Compacting` via `Bus::set_state`, which emits
   `TurnStateChanged`.
-- `POST /api/effort` (`effort=<level>`) — switch the claude
-  effort level for future sessions. Validated server-side against
-  `EFFORT_LEVELS` (`low`/`medium`/`high`/`xhigh`/`max`) — 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).
-- On an ACP agent, `/api/effort` accepts only values the session offers,
-  and the runtime sets the level with `session/set_config_option` before
-  the next prompt.
 - `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
@@ -317,7 +303,7 @@ shaped).
   the frontend (`useAgentState.ts`) on page load and, on a poll, while
   `status === 'needs_login_in_progress'`. Includes `turn_state`,
   `context_window_tokens`, `qualified_label`,
-  `model`, `model_source`, `effort`, `available_efforts`, `links`, and
+  `model`, `model_source`, `effort`, `effort_source`, `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
@@ -391,9 +377,8 @@ Bus events (new vocabulary on `/events/stream`):
   429 response and clears it when the retry sleep expires.
 - `model_changed { model }` — drives the model chip. The harness emits it
   when it applies or clears a swarm value.
-- `effort_changed { effort }` — drives the effort picker chip.
-  Emitted from `Bus::set_effort` on `POST /api/effort`; applies on
-  the next session start.
+- `effort_changed { effort }` — drives the effort chip. The harness
+  emits it when it applies or clears a swarm value.
 - `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