docs(autoagents): add comprehensive implementation summary

Create AUTOAGENTS_IMPLEMENTATION_SUMMARY.md documenting:

## Completed Work (Tasks 1-16 of 18)
- Phase 1: Foundation (dependency, module structure)
- Phase 2: LLM Provider Bridge (ChatProvider, tier plugin)
- Phase 3: Tool Layer (6 inner AutoAgents tools)
- Phase 4: Agent Definition (macros, output types)
- Phase 5: Orchestration (builder, executor)
- Phase 6: MCP Integration (feature-gated workflow)
- Phase 7: Build & Documentation (Makefile, guides)

## Architecture
- Complete component hierarchy diagram
- File structure breakdown
- Two-layer tool architecture explanation

## Technical Decisions
- Feature flag strategy (opt-in experimental)
- Async-to-sync adapter pattern
- Deprecation without removal timeline

## Lessons Learned
- API discovery challenges
- Derive macro constraints (i32/i64/String only)
- Async complexity and runtime bridges
- Import path confusion

## Known Issues & Next Steps
- Tier detection (defaults to Medium, needs implementation)
- Progress notifications (not integrated yet)
- Output format alignment (answer vs final_answer)
- Testing pending (Tasks 17-18)

## Risk Assessment
- Low risk: Feature isolation, fallback available
- Medium risk: Performance, output format
- High risk mitigated: Framework dependency pinned

Total: 12 commits implementing core AutoAgents integration.
Ready for testing phase once compilation verified.

Part of Task 18 from AutoAgents integration revised plan.

Author: jruokola