Capture git diff/status context when workspace is a git repository

[bme2003]

Summary

Workspace snapshots currently fall back to filesystem-based context. For real git repos, we should capture git-specific evidence too.

Why

This gives better context for summaries, handoffs, and future search.

Proposed Change

• Capture git status --short
• Capture changed file list
• Optionally store a truncated git diff --stat or diff preview
• Only do this when the workspace is a valid git repo

Acceptance Criteria

• Git repos produce richer snapshot events
• Non-git directories still work
• Resume and handoff flows can surface git-changed files

[IlyasFardaouix]

I think the issue here is that we're not checking if the workspace is a git repository before falling back to filesystem-based context. We should add a check to see if the current working directory is a git repository using git rev-parse --is-inside-work-tree before attempting to capture git-specific evidence. This will ensure we only try to capture git context when it's actually available.