feat(chat): show skills list on $ trigger for Codex instead of experts

The Codex $ prefix autocomplete now lists skills from
acp_list_agent_skills (excluding ids that match built-in experts), while
experts remain reachable via the dedicated Sparkles button. Non-Codex
agents are unchanged and still show only their native ACP commands on
the / trigger.

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

src/hooks/use-agent-skills.ts

@@ -0,0 +1,87 @@
+"use client"
+
+import { useCallback, useEffect, useMemo, useState } from "react"
+
+import { acpListAgentSkills } from "@/lib/api"
+import type { AgentSkillItem, AgentType } from "@/lib/types"
+
+const agentCache = new Map<AgentType, AgentSkillItem[]>()
+const inflightMap = new Map<AgentType, Promise<AgentSkillItem[]>>()
+
+const EMPTY: AgentSkillItem[] = []
+
+function fetchForAgent(agentType: AgentType): Promise<AgentSkillItem[]> {
+  let promise = inflightMap.get(agentType)
+  if (!promise) {
+    promise = acpListAgentSkills({ agentType })
+      .then((result) => {
+        const skills = result.supported ? result.skills : EMPTY
+        agentCache.set(agentType, skills)
+        inflightMap.delete(agentType)
+        return skills
+      })
+      .catch((err) => {
+        inflightMap.delete(agentType)
+        console.warn("[useAgentSkills] failed:", err)
+        return EMPTY
+      })
+    inflightMap.set(agentType, promise)
+  }
+  return promise
+}
+
+export function useAgentSkills(agentType: AgentType | null): AgentSkillItem[] {
+  const cached = useMemo(
+    () => (agentType ? (agentCache.get(agentType) ?? null) : null),
+    [agentType]
+  )
+  // Track which agent type the fetched result belongs to so stale data
+  // from a previous agent is never returned after a switch.
+  const [fetched, setFetched] = useState<{
+    agentType: AgentType
+    skills: AgentSkillItem[]
+  } | null>(null)
+
+  const doFetch = useCallback(() => {
+    if (!agentType || agentCache.has(agentType)) return
+    let cancelled = false
+    fetchForAgent(agentType).then((list) => {
+      if (!cancelled) setFetched({ agentType, skills: list })
+    })
+    return () => {
+      cancelled = true
+    }
+  }, [agentType])
+
+  // Initial fetch
+  useEffect(() => doFetch(), [doFetch])
+
+  // Re-fetch when window regains focus (covers cross-window cache
+  // invalidation — e.g. settings window creates/removes skills while the
+  // conversation window stays mounted).
+  useEffect(() => {
+    const onFocus = () => {
+      if (!agentType) return
+      agentCache.delete(agentType)
+      inflightMap.delete(agentType)
+      doFetch()
+    }
+    window.addEventListener("focus", onFocus)
+    return () => window.removeEventListener("focus", onFocus)
+  }, [agentType, doFetch])
+
+  if (!agentType) return EMPTY
+  if (cached) return cached
+  if (fetched && fetched.agentType === agentType) return fetched.skills
+  return EMPTY
+}
+
+export function invalidateAgentSkillsCache(agentType?: AgentType) {
+  if (agentType) {
+    agentCache.delete(agentType)
+    inflightMap.delete(agentType)
+  } else {
+    agentCache.clear()
+    inflightMap.clear()
+  }
+}