feat: add a permission-gated UI action registry and action slots for plugins

Moves the terminal toolbar's hardwired AI button into a slot the ssh-terminal
plugin offers and the ai plugin fills, gated on ai.services.use.

src/ui/hooks/use-action-slot.ts

@@ -0,0 +1,37 @@
+import { useMemo, useSyncExternalStore } from "react";
+import {
+  getSlotContributions,
+  subscribeToActionRegistry,
+  getActionPermission,
+  type SlotContribution,
+} from "@/shell/action-registry";
+import { usePermissions } from "@/hooks/use-permissions";
+
+/**
+ * The contributions in a slot that the current user may actually see.
+ *
+ * A slot owner calls this without knowing which plugins contributed, or
+ * whether any did. Contributions the user lacks the permission for are
+ * dropped entirely rather than disabled, which is how admin-gated UI already
+ * behaves elsewhere in the app.
+ */
+export function useActionSlot(slotId: string): SlotContribution[] {
+  const contributions = useSyncExternalStore(
+    subscribeToActionRegistry,
+    () => getSlotContributions(slotId),
+    () => getSlotContributions(slotId),
+  );
+
+  const { has, loaded } = usePermissions();
+
+  return useMemo(() => {
+    // Render nothing until the grants are known. A button that appears and
+    // then vanishes is worse than one that appears a moment late.
+    if (!loaded) return [];
+    return contributions.filter((contribution) => {
+      const permission =
+        getActionPermission(contribution.actionId) ?? undefined;
+      return !permission || has(permission);
+    });
+  }, [contributions, has, loaded]);
+}