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'
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