Editor: Phase 6 - Polish & Testing

[neumaennl]

Summary:
Final testing, performance, documentation, and UX refinement based on real-world usage. (ADR 001)

Related ADR:
docs/architecture/001-editor-transition.md — Phase 6

Acceptance criteria:

• Integration/performance tests for all core flows
• Error handling is robust and user-friendly
• Accessibility and UX passes review
• User/documentation up to date

Tasks:

• Full integration tests for editing flows
• Robust error handling in both extension and webview
• Accessibility pass and UX review
• Documentation written for end-users and contributors