pi_agent: loop + Agent SDK with events and tool hooks

[maozhen520]

Parent

• Spec: #18
• Handoff: #17

What to build

End-to-end agent runtime behavior behind a fake StreamFn: pure loop + stateful Agent with prompt/continue/steer/follow_up, awaited subscribe settlement, fixed event sequence, AgentMessage→LLM Message projection, and tool before/after hooks under parallel|sequential batching.

Acceptance criteria

• Dual public surface: agent loop and stateful Agent
• Agent supports prompt / continue / steer / follow_up without concurrent prompt
• Awaited ordered subscribe settles handlers before the runtime advances
• Full agent/turn/message/tool event set emits in the settled order
• Transcript stores AgentMessages; LLM Messages appear only at stream time via convert hooks
• Tool batching supports parallel and sequential modes with before/after hooks
• Stream function is injectable (no hard dependency on live LiteLLM inside tests)
• Pytest verifies the above with a fake StreamFn

Blocked by

• pi_llm: LiteLLM stream, tools assemble, credentials, errors

[maozhen520]

Implemented on branch implement/21-pi-agent (ea15e6d).

• Dual surface: agent_loop / agent_loop_continue + stateful Agent
• prompt / continue / steer / follow_up; concurrent prompt rejected
• Awaited ordered subscribe settlement (including serialized parallel tool emits)
• Full event sequence + tool hooks under parallel|sequential; injectable fake StreamFn
• Pytest: 12 new package tests (workspace suite green)