* feat(dashboard): two-column layout with redesigned widget
- Two-column layout: progress bar left, task checklist right
- 4 widget modes: full → small → min → off (cycle with /gsd widget)
- Health indicator and ETA in header line for immediate visibility
- Simplified stats: 3 items (hit rate, cost, context %) instead of 7
- Short PWD (last 2 segments), git worktree name with ⎇ prefix
- Last commit time + message in footer (cached every 15s)
- Preview script with mock data for all modes
* docs: add dashboard widget screenshots for PR #1530
* docs: update dashboard screenshots with wider renders
* docs: wider full-width dashboard screenshots
* feat(dashboard): persist widget_mode in preferences
- Add widget_mode to GSDPreferences and KNOWN_PREFERENCE_KEYS
- Load saved widget_mode from preferences on first access
- Persist to global PREFERENCES.md on /gsd widget change
- Default remains "full" when no preference is set
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