feat(dashboard): add skill autocomplete server-side enumeration and client wiring
- Add SkillsMixin with _enumerate_claude_skills and _enumerate_codex_skills
- Claude: reads ~/.claude/skills/, parses YAML frontmatter for descriptions
- Codex: reads curated cache + ~/.codex/skills/ user directory
- Add /api/skills?agent= endpoint to HttpMixin
- Add fetchSkills() API helper in dashboard
- Wire autocomplete config through Modal -> SessionCard -> SimpleInput
- Add getTriggerInfo() for detecting trigger at valid positions
Closes: bd-3q1, bd-sv1, bd-3eu, bd-g9t, bd-30p, bd-1ba, bd-2n7, bd-3s3
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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