docs: add systematic failure mode analysis and training strategy

Comprehensive analysis of GUI agent failure modes with taxonomy,
recording system design, training viability assessment, and
prioritized action plan. Key findings:

- 4-category taxonomy: Environment, Agent Planning, Grounding, Verifier
- Existing ExecutionTraceCollector needs only minor extensions
- SFT on 50-100 corrected trajectories expected 10-30pp improvement
- Deterministic infrastructure fixes should come first (Tier 1)

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

Author: abrichr