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>

dashboard/components/Modal.js

@@ -1,14 +1,29 @@
 import { html, useState, useEffect, useCallback } from '../lib/preact.js';
 import { SessionCard } from './SessionCard.js';
+import { fetchSkills } from '../utils/api.js';
 
 export function Modal({ session, conversations, onClose, onRespond, onFetchConversation, onDismiss }) {
   const [closing, setClosing] = useState(false);
+  const [autocompleteConfig, setAutocompleteConfig] = useState(null);
 
   // Reset closing state when session changes
   useEffect(() => {
     setClosing(false);
   }, [session?.session_id]);
 
+  // Load autocomplete skills when agent type changes
+  useEffect(() => {
+    if (!session) {
+      setAutocompleteConfig(null);
+      return;
+    }
+
+    const agent = session.agent || 'claude';
+    fetchSkills(agent)
+      .then(config => setAutocompleteConfig(config))
+      .catch(() => setAutocompleteConfig(null));
+  }, [session?.agent]);
+
   // Animated close handler
   const handleClose = useCallback(() => {
     setClosing(true);
@@ -54,6 +69,7 @@ export function Modal({ session, conversations, onClose, onRespond, onFetchConve
           onRespond=${onRespond}
           onDismiss=${onDismiss}
           enlarged=${true}
+          autocompleteConfig=${autocompleteConfig}
         />
       </div>
     </div>