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/termActions.ts

@@ -1,6 +1,5 @@
-// Same-origin fetch POST helpers for the term-input slash commands —
-// same shape as modelEffort.ts's `post()` ({ ok, detail } rather than
-// throwing), reused by TermInput.tsx.
+// Same-origin fetch POST helpers for the term-input slash commands
+// ({ ok, detail } rather than throwing), reused by TermInput.tsx.
 async function post(
   path: string,
   body?: URLSearchParams,