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/agent.rs

@@ -2,7 +2,7 @@ use super::state::StateManager;
 use super::structured;
 use super::types::AgentNode;
 use crate::config::RequestContext;
-use crate::function::supervisor::run_agent_for_graph;
+use crate::function::agents::run_agent_for_graph;
 use anyhow::{Context, Result};
 use serde_json::Value;
 use std::time::Duration;