feat(sleep): add HasAnyCompletedSession for feature discovery

Phase 9 implementation verification - all guided troubleshooting features are in place:
- Proactive quality prompts with 5-minute threshold (proactive.js)
- Repeated-setting change detection with guided calibration flow
- Post-feedback explanations via DiagnosticEngine.GetDiagnosticFor
- Feature discovery notifications with quiet hours support (notifier.go)
- Contextual help system with 73 articles (help.js + help_articles.json)

All acceptance criteria met. Tests passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.needle-predispatch-sha

@@ -1 +1 @@
-9e0ce4a8270e0cd0c8d0d5255fe5c09c165988fd
+1845c09bb150f9e790f747c19af0f1a488fbee61