docs(tinyclaw): Phase 2 Research and Design Documents

- Add Phase 2 Research Document (tinyclaw-phase2-research.md)
  - Problem statement and success criteria
  - Essential questions check (all 3 YES)
  - WhatsApp, voice, and skills analysis
  - Risk assessment and mitigation strategies
  - Dependencies and alternatives considered

- Add Phase 2 Design Document (tinyclaw-phase2-design.md)
  - Architecture with component diagrams
  - File changes: 6 new files, 6 modified
  - API design for Matrix channel, voice tool, skills system
  - Test strategy with unit/integration/property tests
  - 6-step implementation plan (4-6 weeks)
  - Performance targets and benchmarks

Phase 2 Scope:
1. WhatsApp via Matrix bridge (Week 1-2)
2. Voice transcription with Whisper (Week 2-3)
3. Skills system (Week 3-5)
4. Integration & polish (Week 5-6)

Part of: #519

Author: AlexMikhalev