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/graph/llm.rs

@@ -6,9 +6,9 @@ use crate::config::prompts::DEFAULT_SKILL_INSTRUCTIONS;
 use crate::config::{
     Input, RequestContext, Role, RoleLike, SkillPolicy, should_inject_skill_instructions,
 };
+use crate::function::agents::{GuardrailAction, check_pending_tasks_guardrail};
 use crate::function::jobs::reap_jobs;
 use crate::function::skill::skill_function_declarations;
-use crate::function::supervisor::{GuardrailAction, check_pending_tasks_guardrail};
 use crate::utils::create_abort_signal;
 use anyhow::{Context, Error, Result, anyhow, bail};
 use log::warn;