refactor(function): finish supervisor-to-agent vocabulary migration

The supervisor registry went kind-generic (TaskHandle::Agent | Job)
earlier in this branch, but the module holding the agent__* handlers
and two model-facing error strings still carried the old name:

- src/function/supervisor.rs -> src/function/agents.rs (it contains
  only agent__* tool handlers, pairing with function/jobs.rs; the
  kind-generic src/supervisor/ registry keeps its name)
- 'Supervisor tool failed' -> 'Agent tool failed'
- 'Unknown supervisor action' -> 'Unknown agent action'

src/acp/server.rs

@@ -1,7 +1,7 @@
 use super::types::{METHOD_NOT_FOUND, PARSE_ERROR, Request, Response};
 use crate::client::call_chat_completions_streaming;
 use crate::config::{Input, RenderMode, RequestContext};
-use crate::function::supervisor::{GuardrailAction, check_pending_tasks_guardrail};
+use crate::function::agents::{GuardrailAction, check_pending_tasks_guardrail};
 use crate::utils;
 use crate::utils::AbortSignal;
 use anyhow::Result;