Extract `llm-planner.ts` into standalone headless `agent-sdk/`

[pAulseperformance]

Epic: Phase 1b Architecture

The architecture document states the Agent SDK should be a headless client library (src/agent-sdk/index.ts). Currently, our AI generation logic lives in src/server/llm-planner.ts and is tightly bound to the Worker.

Acceptance Criteria:

• llm-planner.ts decoupled from Worker context.
• AI Agents can run as independent Node CLI processes connecting via WebSocket.
• Clean public API for the SDK.