feat(app): add skill system

[comppaz]

Summary

• Add complete skill system with 3 built-in skills (Extract Action Items, Generate Insights, Export to Markdown), full CRUD for custom skills, and .think-skill import/export format
• Add trigger engine for automatic skill execution on memory save with condition-based rules (field/operator/value, AND/OR logic) and live preview with match ratios
• Add chat integration with pattern-based skill suggestions and SSE-streamed in-chat execution results
• Add SkillsPage with browse/history tabs, SkillDetail panel, SkillEditor, SkillRunner, and execution history with filtering

Test plan

• Open Skills page → verify built-in skills are listed with correct categories and icons
• Click a skill → verify detail panel shows description, prompt preview, and automation section
• Create a custom skill via editor → verify it appears in the list
• Run a skill on a memory → verify SSE streaming output and result saved to execution history
• Add a trigger with conditions → verify preview shows matching memory count
• Save a memory matching trigger conditions → verify skill executes automatically
• Send chat message with source memories containing "key takeaways" → verify skill suggestion chips appear
• Click a skill chip in chat → verify streamed result renders with gradient header and footer link
• Verify dark mode for all new components