[FEATURE] Git-diff aware incremental re-index (replace file-watcher polling)

[Wolfvin]

Problem

Current watch command uses watchdog file events. This is fine but:

• Re-parses entire file on any change (not just changed functions)
• No awareness of git state — re-indexes uncommitted scratch work
• Doesn't handle branch switches (checkout rewrites many files at once)

Proposed Change

Replace/augment with a git-diff based watcher:

1. On session start: git diff HEAD --name-only to get changed files since last index
2. Only re-parse changed files
3. On git checkout detection (HEAD hash change): trigger targeted re-index of switched files
4. Store last-indexed git SHA in registry metadata

Bonus: detect_changes upgrade

The existing detect_changes / diff commands can be enhanced to use this same git-aware delta to show agents exactly which symbols changed and what their downstream impact is.

Expected Result

Agents working in active git repos always have a fresh, accurate index without wasting compute re-parsing unchanged files.