feat(cli): gd compare and AI variance diagnosis (2/3)

[psiarkiewicz]

Summary

Part 2 of 3 (stacked on #1186 ).

Adds headless CLI run comparison and multi-stage LLM variance diagnosis.

• CLI: gd compare <runDirA> <runDirB>.
• GCS Downloader: Lazy downloading and caching of remote runs with streaming logs.
• Comparison Engine: Diffs files against base apps, compares Playwright results, and aligns trajectory steps.
• Diagnosis Subagents: Multi-stage LLM analysis (compliance auditor, code friction analyzer, synthesis engine) outputting variance_diagnosis.md.