fix: Resolve Google Calendar password prompts via advice

Fixed oauth2-auto.el caching bug using Emacs advice system (survives updates).

Root Cause:
- oauth2-auto version 20250624.1919 has `or nil` on line 206
- This completely disables the internal hash-table cache
- Every org-gcal sync requires decrypting oauth2-auto.plist from disk
- GPG passphrase prompted every ~15 minutes (violated "Frictionless" value)

The Fix (via advice):
- Created cj/oauth2-auto--plstore-read-fixed with cache enabled
- Applied as :override advice to oauth2-auto--plstore-read
- Survives package updates (unlike direct modification)
- Can be easily removed if upstream fixes the bug

Changes:
- modules/auth-config.el:
  * Added cj/oauth2-auto--plstore-read-fixed (lines 75-93)
  * Applied advice on package load (lines 96-98)
  * Added cj/clear-oauth2-auto-cache helper
  * Documented fix in commentary (lines 16-22)
- todo.org: Mark #A priority task as DONE
- docs/oauth2-auto-cache-fix.md: Detailed documentation

Result:
- Passphrase prompted ONCE per Emacs session (on cold start)
- Subsequent org-gcal syncs use cached tokens (no prompts)
- Workflow now frictionless as intended
- Fix persists across package updates

Upstream:
- Bug acknowledged in code: "Assume cache is invalidated. FIXME"
- Should report to: https://github.com/rhaps0dy/emacs-oauth2-auto
- Simple fix: Remove `or nil` on line 206

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

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

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