Refactor agent runtime: state machine, feedback processing, execution log

- Add state.rs with AgentState/Step/StepStatus/AgentPhase as single source of truth
- Extract prompts to markdown files loaded via include_str!
- Replace plan_steps table with execution_log + agent_state_snapshots
- Implement user feedback processing with docker-build-cache plan diff:
  load snapshot → LLM revise_plan → diff (title, description) → invalidate from first mismatch → resume
- run_agent_loop accepts optional initial_state for mid-execution resume
- Broadcast plan step status (done/running/pending) to frontend on step transitions
- Rewrite frontend types/components to match new API (ExecutionLogEntry, PlanStepInfo with status)

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

src/main.rs

@@ -4,6 +4,7 @@ mod db;
 mod kb;
 mod llm;
 mod exec;
+mod state;
 mod timer;
 mod ws;