docs: add detailed implementation tracker with 8 phases and 35 checkbox tasks
Derived from GAP_ANALYSIS.md (25 gaps across 5 categories).
Structured as 8 sequential phases with:
- Per-task checkboxes with gap IDs for traceability
- Specific file paths and code patterns for each fix
- Verification commands per phase
- Commit message conventions
- Test target summary (30→132 tests across all surfaces)
- Progress log table for tracking commits
- Deferred items (B4 webhooks, B5 sharing, E2 WINDSURF_CONTEXT)
Phases: 1-Bug Fixes, 2-Code Quality, 3-Backend Tests,
4-Web Features, 5-DevOps, 6-E2E+Mobile Tests,
7-AI Enrichment, 8-Docs Alignment
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