feat(web): /skill slash command with autocomplete

Trigger /<name>, dropdown lists all skills filtered by name prefix,
arg passthrough sends the rest as the user message. Synthetic
skill_use tool_use renders identically to model-invoked skills.

apps/web/src/components/SkillSlashCommand.tsx

@@ -0,0 +1,137 @@
+import { useEffect, useMemo, useRef, useState } from 'react';
+import { cn } from '@/lib/utils';
+import type { Skill } from '@/api/types';
+
+interface Props {
+  query: string;
+  skills: Skill[];
+  anchorRect: { top: number; left: number };
+  onSelect: (skillName: string) => void;
+  onClose: () => void;
+}
+
+// Batch 9.6: slash-command dropdown. Models FileMentionPopover's pattern —
+// fixed-positioned popover, keyboard nav, click-outside-to-close. shadcn
+// `Command` (cmdk) isn't installed in this project; per the addendum we use
+// a plain div + Tailwind instead of pulling a new primitive autonomously.
+
+// Case-insensitive prefix match on `name` only. Description is display-only
+// in v1 (substring search across description is deferred to a polish batch).
+function filterByPrefix(skills: Skill[], query: string): Skill[] {
+  const q = query.toLowerCase();
+  const filtered = q
+    ? skills.filter((s) => s.name.toLowerCase().startsWith(q))
+    : skills;
+  // Stable alphabetical ordering matches the server's cache order (skills.ts
+  // sorts on name asc) but we re-sort here so a stale client cache doesn't
+  // surprise the user.
+  return [...filtered].sort((a, b) => a.name.localeCompare(b.name));
+}
+
+export function SkillSlashCommand({ query, skills, anchorRect, onSelect, onClose }: Props) {
+  const [highlightIndex, setHighlightIndex] = useState(0);
+  const popoverRef = useRef<HTMLDivElement>(null);
+  const filtered = useMemo(() => filterByPrefix(skills, query), [skills, query]);
+
+  useEffect(() => { setHighlightIndex(0); }, [query]);
+
+  // Arrow / Enter / Tab / Escape. Bound on document so keystrokes from the
+  // textarea reach the popover even though focus stays in the textarea.
+  useEffect(() => {
+    function handleKeyDown(e: KeyboardEvent) {
+      if (e.key === 'ArrowDown') {
+        e.preventDefault();
+        setHighlightIndex((prev) => (prev < filtered.length - 1 ? prev + 1 : 0));
+      } else if (e.key === 'ArrowUp') {
+        e.preventDefault();
+        setHighlightIndex((prev) => (prev > 0 ? prev - 1 : filtered.length - 1));
+      } else if (e.key === 'Enter' || e.key === 'Tab') {
+        if (filtered.length === 0) return;
+        e.preventDefault();
+        const target = filtered[highlightIndex] ?? filtered[0];
+        if (target) onSelect(target.name);
+      } else if (e.key === 'Escape') {
+        e.preventDefault();
+        onClose();
+      }
+    }
+    document.addEventListener('keydown', handleKeyDown);
+    return () => document.removeEventListener('keydown', handleKeyDown);
+  }, [filtered, highlightIndex, onSelect, onClose]);
+
+  useEffect(() => {
+    function handleMouseDown(e: MouseEvent) {
+      if (popoverRef.current && !popoverRef.current.contains(e.target as Node)) {
+        onClose();
+      }
+    }
+    document.addEventListener('mousedown', handleMouseDown);
+    return () => document.removeEventListener('mousedown', handleMouseDown);
+  }, [onClose]);
+
+  useEffect(() => {
+    const el = popoverRef.current?.querySelector('[data-highlighted="true"]');
+    if (el) el.scrollIntoView({ block: 'nearest' });
+  }, [highlightIndex]);
+
+  // Anchor sits above the input — translate(-100%) on Y so the dropdown
+  // expands upward from the anchor point rather than over the textarea.
+  const style = {
+    top: anchorRect.top,
+    left: anchorRect.left,
+    transform: 'translateY(-100%)',
+  } as const;
+
+  if (filtered.length === 0) {
+    return (
+      <div
+        ref={popoverRef}
+        className="fixed z-50 bg-popover border border-border rounded-md shadow min-w-[320px] p-2"
+        style={style}
+      >
+        <div className="text-xs text-muted-foreground px-2 py-1">
+          {query ? `No skill starts with "/${query}"` : 'No skills available'}
+        </div>
+      </div>
+    );
+  }
+
+  return (
+    <div
+      ref={popoverRef}
+      className="fixed z-50 bg-popover border border-border rounded-md shadow min-w-[320px] max-w-[420px] max-h-[320px] overflow-y-auto"
+      style={style}
+    >
+      {filtered.map((skill, i) => (
+        <button
+          key={skill.name}
+          type="button"
+          data-highlighted={i === highlightIndex}
+          className={cn(
+            'w-full text-left px-2.5 py-2 cursor-pointer block',
+            i === highlightIndex && 'bg-muted',
+          )}
+          onMouseEnter={() => setHighlightIndex(i)}
+          onMouseDown={(e) => {
+            // mousedown not click — click runs after blur/focus shuffles which
+            // can race with the textarea's onBlur close path.
+            e.preventDefault();
+            onSelect(skill.name);
+          }}
+        >
+          <div className="font-mono text-xs font-bold text-foreground">/{skill.name}</div>
+          <div
+            className="text-xs text-muted-foreground overflow-hidden"
+            style={{
+              display: '-webkit-box',
+              WebkitLineClamp: 2,
+              WebkitBoxOrient: 'vertical',
+            }}
+          >
+            {skill.description}
+          </div>
+        </button>
+      ))}
+    </div>
+  );
+}