feat(spec): add llm() action-backend marker

pkg/spec/src/action-tree.ts

@@ -57,10 +57,13 @@ export type ActionDescriptor =
 //   weighted: probabilistic choice over child nodes, scanned ascending.
 //   actions:  author callback returning a list to pick uniformly from.
 //   builtin:  host-backed leaf identified by a verb.
+//   llm:      marker selecting the LLM action backend; inert on the JS picker
+//             (walk returns null), Go reads config.model off globalThis.actions.
 export type GeneratorNode =
   | { kind: "weighted"; branches: ReadonlyArray<readonly [number, GeneratorNode]> }
   | { kind: "actions"; generate: () => ActionDescriptor[] }
-  | { kind: "builtin"; verb: BuiltinVerb };
+  | { kind: "builtin"; verb: BuiltinVerb }
+  | { kind: "llm"; config: { model: string } };
 
 // Candidate is one host-enumerated target for a builtin verb. The host
 // resolves geometry (and a native selector) so no element handle crosses into

pkg/spec/src/actions.ts

@@ -34,6 +34,15 @@ export function actions(generator: () => Action[]): GeneratorNode {
   return { kind: "actions", generate: generator as () => ActionDescriptor[] };
 }
 
+// llm selects the LLM action backend: instead of the seeded picker drawing a
+// random candidate, Go drives an OpenRouter model that chooses which candidate
+// to act on from the screenshot + the candidate list. The returned marker is
+// inert on the JS picker (pick.ts walks it to null); Go reads its config.model
+// off globalThis.actions. API key comes from OPENROUTER_API_KEY.
+export function llm(config: { model: string }): GeneratorNode {
+  return { kind: "llm", config };
+}
+
 export function whenRoute(
   routeExtractor: { readonly current: string | null },
   routes: string | readonly string[],

pkg/spec/src/index.ts

@@ -42,6 +42,7 @@ export {
   actions,
   doubleTaps,
   from,
+  llm,
   longPresses,
   pressKeys,
   scrolls,