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

frontend/packages/agent/src/lib/pauseAction.ts

@@ -5,8 +5,7 @@
 // origin, but accessed directly they're different ports) and this way
 // hive-c0re doesn't need to grow CORS headers for it — the tradeoff is
 // an opaque, unreadable response, which is fine here since the caller
-// doesn't need to read it, only trigger a state refresh afterward (same
-// shape as `postEffort` in modelEffort.ts). `credentials:
+// doesn't need to read it, only trigger a state refresh afterward. `credentials:
 // 'include'` matches the cookie-sending behavior of the plain <form>
 // submit this replaces.
 //