fix: capture mounted workspace diffs

[chubes4]

Summary

• Compare repo-backed mounted workspace copies against their baseline directories during artifact capture without requiring git inside Playground.
• Record per-mount diff extraction status in files/diffs.json, including skipped mounts without baselines and failed extraction diagnostics.
• Add a focused smoke test covering simulated workspace_write additions and workspace_edit modifications/deletions.

Closes #429.

Testing

• npm run mounted-workspace-diff-smoke
• npm run build
• npm run artifact-contract-smoke
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the mounted workspace diff artifact capture update, added focused smoke coverage, and ran targeted verification. Chris remains responsible for review and merge.