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

swarm-queue-client/src/agent_inference.rs

@@ -81,8 +81,8 @@ pub enum Source {
     Swarm,
     /// `services.hyperhive.agent.model` or `effortLevel` in its agent.nix.
     AgentNix,
-    /// `hyperhive-effort`, written by an `/effort` pick in its web UI, or a
-    /// `hyperhive-model` file, which the agent only reads.
+    /// A `hyperhive-model` or `hyperhive-effort` file in its harness dir, which
+    /// the agent only reads.
     LocalFile,
     /// The harness's built-in default.
     Default,