feat: Implement date tracking for skills

- Added `date_added` field to all skills in frontmatter.
- Updated Home page to display skill addition date alongside risk level.
- Enhanced Skill Detail page to show date added in a badge format.
- Created scripts for managing skill dates, including adding missing dates and generating reports.
- Updated validators to enforce date format compliance.
- Added comprehensive documentation on date tracking implementation and usage.
- Introduced a new skill template including the `date_added` field.

scripts/validate-skills.js

@@ -41,6 +41,7 @@ const ALLOWED_FIELDS = new Set([
   "metadata",
   "allowed-tools",
   "package",
+  "date_added",
 ]);
 
 const USE_SECTION_PATTERNS = [