fix(chat): query expert skills via symlinks and use $ prefix for Codex

Expert skills in the chat session were derived by intersecting built-in
experts with ACP availableCommands, which caused Codex experts to never
appear since Codex does not advertise skills through ACP.

- Add `experts_list_for_agent` backend API that checks symlink status
  across all global skill dirs for the given agent type
- Replace availableCommands-based expert filtering with symlink-based
  query, making the settings page the single source of truth
- Use `$` prefix for Codex expert skills while keeping `/` for slash
  commands and other agents' experts
- Disable the expert button when no experts are linked for the agent
- Invalidate per-agent expert cache after link/unlink in settings

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

src/hooks/use-agent-experts.ts

@@ -0,0 +1,68 @@
+"use client"
+
+import { useEffect, useMemo, useState } from "react"
+
+import { expertsListForAgent } from "@/lib/api"
+import type { AgentType, ExpertListItem } from "@/lib/types"
+
+const agentCache = new Map<AgentType, ExpertListItem[]>()
+const inflightMap = new Map<AgentType, Promise<ExpertListItem[]>>()
+
+const EMPTY: ExpertListItem[] = []
+
+export function useAgentExperts(agentType: AgentType | null): ExpertListItem[] {
+  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
+    experts: ExpertListItem[]
+  } | null>(null)
+
+  useEffect(() => {
+    if (!agentType || agentCache.has(agentType)) return
+
+    let cancelled = false
+    let promise = inflightMap.get(agentType)
+    if (!promise) {
+      promise = expertsListForAgent(agentType)
+        .then((list) => {
+          agentCache.set(agentType, list)
+          inflightMap.delete(agentType)
+          return list
+        })
+        .catch((err) => {
+          inflightMap.delete(agentType)
+          console.warn("[useAgentExperts] failed:", err)
+          return EMPTY
+        })
+      inflightMap.set(agentType, promise)
+    }
+
+    promise.then((list) => {
+      if (!cancelled) setFetched({ agentType, experts: list })
+    })
+
+    return () => {
+      cancelled = true
+    }
+  }, [agentType])
+
+  if (!agentType) return EMPTY
+  if (cached) return cached
+  if (fetched && fetched.agentType === agentType) return fetched.experts
+  return EMPTY
+}
+
+export function invalidateAgentExpertsCache(agentType?: AgentType) {
+  if (agentType) {
+    agentCache.delete(agentType)
+    inflightMap.delete(agentType)
+  } else {
+    agentCache.clear()
+    inflightMap.clear()
+  }
+}