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/function/jobs.rs

@@ -1,7 +1,7 @@
+use super::agents::AGENT_FUNCTION_PREFIX;
 use super::memory::MEMORY_FUNCTION_PREFIX;
 use super::rag_query::RAG_FUNCTION_PREFIX;
 use super::skill::SKILL_FUNCTION_PREFIX;
-use super::supervisor::AGENT_FUNCTION_PREFIX;
 use super::todo::TODO_FUNCTION_PREFIX;
 use super::user_interaction::USER_FUNCTION_PREFIX;
 use super::{FunctionDeclaration, JsonSchema, PATH_SEP, mcp_error_display, render_tool_result};
@@ -1217,7 +1217,7 @@ fn tail_chars(text: &str, max_chars: usize) -> Option<String> {
 mod tests {
     use super::*;
     use crate::config::{AppConfig, AppState, WorkingMode};
-    use crate::function::supervisor::{
+    use crate::function::agents::{
         GuardrailAction, check_pending_tasks_guardrail, handle_agent_tool,
     };
     use crate::supervisor::mailbox::Inbox;