Consolidate Plan Scanner with Shared Git Timestamp Utility

Summary

Refactor the plan scanner's getGitMtimes() in runes/plan/src/scanner.ts to use the shared git timestamp utility from packages/content/, eliminating duplicate git timestamp collection code. The plan-activity rune's mtime field will be sourced from the same data as $file.modified, ensuring consistency between the activity feed and individual rune attributes.

Acceptance Criteria

• runes/plan/src/scanner.ts imports and uses the shared timestamp utility from packages/content/ instead of its own getGitMtimes() implementation
• The getGitMtimes() function is removed from scanner.ts
• The mtime field in scanner results is derived from the shared utility's modified value
• plan-activity rune displays the same modification dates as $file.modified on individual plan runes
• No functional change in plan-activity output (dates match previous behavior)
• All existing plan scanner tests pass

Approach

1. Add @refrakt-md/content as a dependency of @refrakt-md/plan (or extract the utility to a shared location both can import)
2. Replace getGitMtimes() calls in scanner.ts with the shared utility
3. Map the shared utility's output format to the scanner's expected mtime field
4. Verify plan-activity output is unchanged

References

• File-Derived Timestamps for Runes (Phase 4 — Plan Scanner Consolidation)
• Create Shared Git Timestamp Utility (shared git timestamp utility — dependency)
• runes/plan/src/scanner.ts — current getGitMtimes() implementation