docs: add Phase 17 master strategy and sprint plan

Comprehensive planning document consolidating all Phase 17 content:
- Executive summary with vision, goals, and scope overview
- Source document references for future Claude Code sessions
- Detailed feature breakdown (Mobile Integration, Push, Voice/Video, Groups)
- Dependency analysis (technical, internal, sprint dependencies)
- Optimized 8-sprint plan with parallel execution tracks
- Technical architecture notes for mobile, voice/video, and groups
- Risk assessment and mitigation strategies
- Success criteria and quality gates

Total estimated scope: 280-370 story points over 17-23 weeks.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: doublegate