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/repl/mod.rs

@@ -16,7 +16,7 @@ use crate::config::{
     StateFlags, flatten_prompt_messages, macro_execute, resolve_prompt_args, sanitize_display_text,
 };
 use crate::config::{AssetCategory, paths};
-use crate::function::supervisor::{GuardrailAction, check_pending_tasks_guardrail};
+use crate::function::agents::{GuardrailAction, check_pending_tasks_guardrail};
 use crate::render::render_error;
 use crate::utils::{
     AbortSignal, SHELL, abortable_run_with_spinner, create_abort_signal, dimmed_text,