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>

amc_server/handler.py

@@ -5,6 +5,7 @@ from amc_server.mixins.control import SessionControlMixin
 from amc_server.mixins.discovery import SessionDiscoveryMixin
 from amc_server.mixins.http import HttpMixin
 from amc_server.mixins.parsing import SessionParsingMixin
+from amc_server.mixins.skills import SkillsMixin
 from amc_server.mixins.state import StateMixin
 
 
@@ -15,6 +16,7 @@ class AMCHandler(
     SessionControlMixin,
     SessionDiscoveryMixin,
     SessionParsingMixin,
+    SkillsMixin,
     BaseHTTPRequestHandler,
 ):
     """HTTP handler composed from focused mixins."""