feat: add /skills command and skill autocomplete

[titouv]

Summary

Adds /skills command and skill autocomplete to improve skill discoverability and invocation.

• /skills command opens a searchable dialog listing all available skills grouped by source (Global/Project)
• Skills appear in / autocomplete as /<skillName> for direct invocation (e.g., /create-plan)
• Selecting a skill invokes it by instructing the LLM to load the skill

Changes

• New: dialog-skill-list.tsx - Dialog component for listing skills
• Modified: app.tsx - Register /skills command
• Modified: autocomplete.tsx - Add skills to slash autocomplete

Testing

1. Run bun dev in packages/opencode
2. Press Ctrl+P, type "skills" to see the command
3. Type /skills to open the skills dialog
4. Type / to see skills in autocomplete alongside other commands

Closes #7846