feat(chat): add experts menu to message input toolbar

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/use-built-in-experts.ts

@@ -0,0 +1,84 @@
+"use client"
+
+import { useEffect, useState } from "react"
+
+import { expertsList } from "@/lib/api"
+import type { ExpertListItem } from "@/lib/types"
+
+// Module-level cache so every MessageInput/ChatInput instance shares a single
+// fetch. Experts are bundled into the binary and change only when codeg is
+// upgraded, so refetching per mount is wasted work.
+let cachedExperts: ExpertListItem[] | null = null
+let inflight: Promise<ExpertListItem[]> | null = null
+const subscribers = new Set<(experts: ExpertListItem[]) => void>()
+
+async function loadExperts(): Promise<ExpertListItem[]> {
+  if (cachedExperts) return cachedExperts
+  if (inflight) return inflight
+  inflight = expertsList()
+    .then((list) => {
+      cachedExperts = list
+      inflight = null
+      for (const subscriber of subscribers) {
+        subscriber(list)
+      }
+      return list
+    })
+    .catch((err) => {
+      inflight = null
+      throw err
+    })
+  return inflight
+}
+
+/**
+ * Returns the list of built-in expert skills bundled into codeg.
+ *
+ * The first call triggers a single backend request; subsequent hook
+ * instances read from an in-memory cache. Safe to call from many components
+ * without causing duplicate fetches.
+ */
+export function useBuiltInExperts(): ExpertListItem[] {
+  const [experts, setExperts] = useState<ExpertListItem[]>(
+    () => cachedExperts ?? []
+  )
+
+  useEffect(() => {
+    // If the cache is already populated the useState initializer above
+    // already handed us the right value — no follow-up setState needed.
+    // Only kick off a fetch when the cache is empty, and always register
+    // the subscriber so concurrent consumers pick up the fresh list the
+    // moment the first load resolves.
+    let cancelled = false
+    if (!cachedExperts) {
+      loadExperts()
+        .then((list) => {
+          if (!cancelled) setExperts(list)
+        })
+        .catch((err) => {
+          console.warn("[useBuiltInExperts] failed to load experts:", err)
+        })
+    }
+
+    const onUpdate = (next: ExpertListItem[]) => {
+      if (!cancelled) setExperts(next)
+    }
+    subscribers.add(onUpdate)
+
+    return () => {
+      cancelled = true
+      subscribers.delete(onUpdate)
+    }
+  }, [])
+
+  return experts
+}
+
+/**
+ * Invalidate the cached experts list. Call this after installing/uninstalling
+ * experts so subsequent consumers see the fresh list.
+ */
+export function invalidateBuiltInExperts(): void {
+  cachedExperts = null
+  inflight = null
+}