Create Shared Git Timestamp Utility

Summary

Create a shared utility in packages/content/ that batch-collects git timestamps (created and modified) for all files in a content directory. This replaces the per-package approach used by runes/plan/src/scanner.ts:getGitMtimes() with a general-purpose solution available to the entire content pipeline.

Acceptance Criteria

• New module in packages/content/ exports a function (e.g. getGitTimestamps) that returns Map<string, { created: string; modified: string }> for all files under a given directory
• Uses batch git log commands (not per-file) for both created and modified timestamps
• Modified timestamp derived from git log --format="%at" --name-only --diff-filter=ACMR HEAD
• Created timestamp derived from git log --format="%at" --name-only --diff-filter=A --reverse HEAD
• Timestamps normalized to ISO 8601 date strings (YYYY-MM-DD)
• Falls back to fs.stat() (birthtimeMs / mtimeMs) when git data is unavailable for a file
• Detects shallow clones via git rev-parse --is-shallow-repository and omits or marks created as unreliable
• Handles non-git directories gracefully (falls back entirely to fs.stat)
• Results cached in memory for the duration of a single loadContent() call (no cross-build caching needed)
• Unit tests covering: normal git repo, shallow clone detection, non-git fallback, date formatting

Approach

1. Create packages/content/src/timestamps.ts with the batch git timestamp collection logic
2. Reuse the proven pattern from runes/plan/src/scanner.ts:getGitMtimes(), extended to also capture creation times
3. Parse git log output into a Map<string, { created: number; modified: number }> keyed by relative file path
4. Add fs.stat fallback for files missing from git history
5. Export a high-level function that returns formatted ISO date strings

References

• File-Derived Timestamps for Runes (Phase 1 — Git Timestamp Utility)
• runes/plan/src/scanner.ts — existing getGitMtimes() implementation to draw from