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/agent-lifecycle/persistence.md

@@ -313,7 +313,8 @@ Under `/var/lib/hyperhive/agents/<name>/`:
     tables above.
   - `hyperhive-events.sqlite` — turn-loop event log.
   - `hyperhive-turn-stats.sqlite` — per-turn timing stats.
-  - `hyperhive-model` — single-line model name file, read-only fallback.
+  - `hyperhive-model` / `hyperhive-effort` — single-line model name and
+    effort level files, read-only fallbacks.
 - `work/` — the agent's working space for git worktrees, build dirs
   and other large working data. Bind-mounted RW to
   `/agents/<name>/work` inside the container and owned by the agent