perf: Cache org-agenda files to improve view performance

Applied same caching strategy as org-refile to org-agenda file building.

Root Cause:
- cj/build-org-agenda-list scanned projects-dir recursively
- Called on EVERY agenda view (F8, C-f8)
- directory-files-recursively scans 3,959 files to find 7 todo.org files
- Contributes to 30+ second agenda view time

Solution Implemented:
1. Cache layer with 1-hour TTL
   - First view: builds and caches file list (one-time cost)
   - Subsequent views: use cache (instant)
   - Auto-refresh after 1 hour or Emacs restart

2. Async cache building
   - Runs 10 seconds after Emacs idle (non-blocking)
   - Zero startup impact
   - Cache ready before first agenda view in typical workflow

3. Manual refresh available
   - M-x cj/org-agenda-refresh-files
   - Use after adding new projects/todo.org files
   - Force rebuild bypasses cache

4. Robust error handling
   - Building flag prevents concurrent builds
   - unwind-protect ensures flag always clears
   - Graceful handling if user views agenda before async build completes

Changes (modules/org-agenda-config.el):
- Added cache variables (lines 85-97)
- Modified cj/build-org-agenda-list for caching (lines 116-161)
- Added cj/org-agenda-refresh-files for manual refresh (lines 171-177)
- Async build via run-with-idle-timer (lines 163-169)
- Enhanced commentary documenting performance (lines 6-12)
- Removed emacs-startup-hook (now using idle timer)

Expected Impact:
- Reduces one component of 30+ second agenda view delay
- File list building: several seconds → instant (cached)
- Combined with org-refile optimization: significant daily time savings

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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