v0.3.0-beta.5

feat: investigate skill, deeper tours, onboarding, hardening

Skill files:
- New investigate-issue.md skill for /side-bae-investigate
- Strengthened generate-tour.md with anti-patterns, self-check, depth enforcement
- Strengthened generate-lesson.md with guided discovery, code-ref rules
- Synced discover-features.md and discover-concepts.md with embedded prompts
- Registered investigate-issue.md in install-skills.ts

Tour quality:
- Orphan node cleanup in validateTourDocument (reachability walk)
- Fixed ordering: reachability before cycle-stripping to preserve convergent nodes
- Added explanation depth self-check to embedded tour prompt

File watcher hardening:
- 300ms debounce per filename (prevents duplicate macOS notifications)
- Distinguish new vs updated tours (no misleading "New tour" on edits)
- Timer cleanup on dispose

Sidebar cache fix:
- refresh() now invalidates all disk-backed caches (features, concepts, changes)
- External writes from skill files now properly update the sidebar

Onboarding:
- viewsWelcome persists until user has real content (returns [] when pristine)
- Auto-open command hub on first install via globalState flag
- Better hub copy and visual divider between primary/secondary actions
- New sideBae.hasContent context key replaces broken featuresLoaded flow

Webview polish:
- Semantic color tokens (--sb-context/problem/solution) replace hardcoded rgba
- ARIA labels on stops, edges, choices, hub actions, loading spinners
- Screen-reader form label for lesson textarea
- Extended reduced-motion coverage (scroll, transforms)
- Defensive guards: lighten() try-catch, null-safe testResults, trail fix

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>