refactor: P0–P5 optimization — library-only, simplified planning, dead code removal

- Remove dead prompt files (10 files: complexity, goal_check, goal_extract, plan, structured_output)
- Remove todo.rs (258 lines) — consolidated into planning module
- Simplify agent.rs — remove server-specific paths, streamline HITL flow
- Simplify session.rs — remove server session management, direct AgentLoop
- Consolidate planning/mod.rs with LlmPlanner (JSON-structured planning)
- Simplify config.rs — remove server/gRPC config fields
- Simplify prompts.rs — remove complexity/goal/plan prompt loaders
- Re-add agent_api.rs (high-level facade: Agent, AgentSession, SessionOptions)
- Clean up queue.rs, hitl.rs, hooks, context_store, tools
- cargo fmt + cargo clippy clean

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ZhiXiao-Lin