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/SimpleInput.js

@@ -1,7 +1,7 @@
-import { html, useState, useRef } from '../lib/preact.js';
+import { html, useState, useRef, useCallback } from '../lib/preact.js';
 import { getStatusMeta } from '../utils/status.js';
 
-export function SimpleInput({ sessionId, status, onRespond }) {
+export function SimpleInput({ sessionId, status, onRespond, autocompleteConfig = null }) {
   const [text, setText] = useState('');
   const [focused, setFocused] = useState(false);
   const [sending, setSending] = useState(false);
@@ -9,6 +9,32 @@ export function SimpleInput({ sessionId, status, onRespond }) {
   const textareaRef = useRef(null);
   const meta = getStatusMeta(status);
 
+  // Detect if cursor is at a trigger position for autocomplete
+  const getTriggerInfo = useCallback((value, cursorPos) => {
+    // No config means no autocomplete
+    if (!autocompleteConfig) return null;
+
+    const { trigger } = autocompleteConfig;
+
+    // Find the start of the current "word" (after last whitespace before cursor)
+    let wordStart = cursorPos;
+    while (wordStart > 0 && !/\s/.test(value[wordStart - 1])) {
+      wordStart--;
+    }
+
+    // Check if word starts with this agent's trigger character
+    if (value[wordStart] === trigger) {
+      return {
+        trigger,
+        filterText: value.slice(wordStart + 1, cursorPos).toLowerCase(),
+        replaceStart: wordStart,
+        replaceEnd: cursorPos,
+      };
+    }
+
+    return null;
+  }, [autocompleteConfig]);
+
   const handleSubmit = async (e) => {
     e.preventDefault();
     e.stopPropagation();