Inject $file.created and $file.modified Variables into Content Pipeline

Summary

Wire the shared git timestamp utility into packages/content/src/site.ts so that every page's Markdoc transform config includes $file.created and $file.modified variables. These join the existing $page and $frontmatter variables, making file timestamps available to any rune via standard Markdoc variable references.

Acceptance Criteria

• loadContent() in packages/content/src/site.ts calls the git timestamp utility once before the page loop
• Each page's contentVariables includes a file object with created and modified string properties
• Three-tier resolution order: frontmatter override > git history > fs.stat fallback
• When no timestamp is available, the variable is undefined (not an empty string or null)
• Variables are accessible in Markdoc schemas as $file.created and $file.modified
• Existing $page and $frontmatter variables are unaffected
• Integration test: a page with no frontmatter dates gets git-derived $file.created and $file.modified
• Integration test: a page with explicit frontmatter created/modified uses those values instead of git data

Approach

1. Import the git timestamp utility in packages/content/src/site.ts
2. Call it once at the start of loadContent() with the content root directory
3. In the per-page loop, look up timestamps for each file path and merge into contentVariables
4. Apply the three-tier fallback: check frontmatter first, then git map, then fs.stat

References

• File-Derived Timestamps for Runes (Phase 2 — Variable Injection)
• Create Shared Git Timestamp Utility (shared git timestamp utility — dependency)
• packages/content/src/site.ts — existing variable injection point