feat(windows): add Git gutter with inline diff peek for editor

[1lck]

Adds visual Git change markers in the editor line number gutter with click-to-peek inline diff viewer for Windows.

What's implemented

Core components:

• GitGutterController: manages gutter decorations, peek widget lifecycle, Monaco integration
• GitGutterPeekWidget: inline diff viewer showing HEAD vs working tree, with prev/next navigation
• git-gutter-changes: computes line-level diff markers (added/modified/deleted) from Git status and file diff
• use-git-gutter-base: React hook providing gutter data and navigation actions

Visual markers:

• Green bar: added lines
• Blue bar: modified lines
• Red triangle: deleted lines

Interaction:

• Click marker → opens inline diff at that change
• Up/Down arrows navigate between changes
• Escape / editor blur / file change → closes widget
• Peek models disposed on close (no leaks)

Settings:

• New toggle git.showGutterChanges (default true) in Git Settings panel
• Search index updated for "gutter" and "inline diff"

Not included (follow-up needed)

• Revert/stage actions in peek widget: blocked on #1015 merge (adds discardHunk API)
• Full test coverage: agent ran out of gateway retries before completing component-level tests and browser validation
• macOS implementation: Windows-only for now
• Platform feature matrix update: needs testing on real Windows machine first

Testing done

• Unit tests for git-gutter-changes and GitGutterPeekSession pass
• Type check passes
• Agent was interrupted by gateway timeouts before completing:
  • Full Vite test suite run
  • Browser visual validation (dev server and harness files were prepared but not executed)
  • Repository verification scripts

What reviewer should check

1. Run full test suite: bun test
2. Run repository checks: verify-windows-boundaries.sh, verify-test-stability.sh, etc.
3. Test on Windows machine: open a Git-tracked file with changes, verify markers appear, click to peek
4. Verify peek widget keyboard navigation (Up/Down arrows between changes)
5. Check that peek models are disposed (no Monaco model leaks)

Refs #681

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