Add Timestamp Attributes to Plan Rune Schemas

Summary

Add created and modified attributes to the plan rune schemas (spec, work, bug, decision, milestone) with defaults referencing $file.created and $file.modified. Update the plan rune configs to display timestamps in rendered output alongside existing status and priority badges.

Acceptance Criteria

• spec, work, bug, decision, and milestone schemas each declare created and modified attributes
• Attribute defaults reference $file.created and $file.modified via Markdoc variable default syntax (or equivalent fallback if Markdoc doesn't support { $variable } in defaults)
• Authors can override timestamps with explicit attribute values (e.g. created="2025-06-15")
• Plan rune configs in runes/plan/src/config.ts updated to render timestamps in the header metadata area
• The decision rune's existing date attribute remains unchanged; created and modified are additive
• Rendered output displays dates in a consistent format alongside existing badges
• Existing plan content that omits created/modified attributes continues to work (attributes are optional)
• Tests updated for new attributes and rendering output

Approach

1. Verify whether Markdoc supports { $variable: 'file.created' } in attribute defaults — if not, resolve defaults during the schema transform phase
2. Add created and modified attributes to each plan rune schema in runes/plan/src/tags/
3. Update runes/plan/src/config.ts to include structure entries for timestamp display
4. Add or update CSS in runes/plan/styles/ for timestamp display elements
5. Update tests and snapshots

References

• File-Derived Timestamps for Runes (Phase 3 — Plan Rune Schemas)
• Inject $file.created and $file.modified Variables into Content Pipeline (variable injection — dependency)
• runes/plan/src/tags/ — plan rune schema files
• runes/plan/src/config.ts — plan rune engine configs