fix(ui): Fix cursor color updates with post-command-hook

The cursor color was not updating correctly when switching buffers
or modifying files. The original implementation used window-buffer-change
and other specific hooks, but these were insufficient and internal
buffers were interfering with cursor color updates.

Root cause:
- Cursor color is global (one cursor for all of Emacs)
- Previous hooks fired in internal buffer contexts (*Echo Area*, *temp*)
- This caused cursor to be set to white (unmodified) even when in
  read-only buffers like dashboard

Solution:
- Use post-command-hook which runs after every command in current buffer
- Ignore internal buffers (names starting with space)
- Cache optimization prevents redundant set-cursor-color calls

Behavior now:
- Dashboard (read-only): Red cursor
- Unmodified file: White cursor
- Modified file: Green cursor
- After save: White cursor

Tests:
- Added 9 integration tests in test-ui-cursor-color-integration.el
- Tests verify hook installation, buffer switching, modification tracking
- All 27 tests passing (18 unit + 9 integration)

Integration tests catch issues that unit tests miss:
- Unit tests verified state detection logic (✓)
- Integration tests verify hooks fire at right times (✓ now)
- Integration tests verify real buffer switching behavior (✓ now)

Cleanup:
- Removed debug code from init.el
- Removed debug-cursor-color.el temporary file

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

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

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