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/config/request_context.rs

@@ -18,11 +18,11 @@ use super::{MessageContentToolCalls, prompts};
 use crate::client::{Model, ModelType, list_models};
 use crate::function::{
     FunctionDeclaration, Functions, ToolCallTracker, ToolResult,
+    agents::AGENT_FUNCTION_PREFIX,
     jobs::{DEFAULT_MAX_CONCURRENT_JOBS, JOB_FUNCTION_PREFIX, is_backgroundable_tool},
     memory::MEMORY_FUNCTION_PREFIX,
     rag_query::RAG_FUNCTION_PREFIX,
     skill::SKILL_FUNCTION_PREFIX,
-    supervisor::AGENT_FUNCTION_PREFIX,
     todo::TODO_FUNCTION_PREFIX,
     user_interaction::USER_FUNCTION_PREFIX,
 };