feat(pr): pr-create/pr-respond/pr-merge node kinds + handlers (U3)

Adds the three first-class PR workflow node kinds and their handlers via
createPrNodeHandlers(deps), registered in createDefaultNodeHandlers
(fail-closed pr-nodes-unwired when absent). GitHub ops are injected as
callbacks (PrNodeGithubOps) at all three CLI sites (daemon/serve/dashboard)
so the engine never imports the dashboard client (FN-3049). pr-create
routes open/failed as outcomes; pr-merge passes expectedHeadOid and never
writes 'merged' (reconcile corroborates); pr-respond delegates to an
injected respond callback (U5 fills the body). 10 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/engine/src/index.ts

@@ -46,6 +46,20 @@ export {
   type CodeNodeRunner,
   type DefaultNodeHandlerDeps,
 } from "./workflow-node-handlers.js";
+export {
+  createPrNodeHandlers,
+  buildPrNodeDeps,
+  type PrNodeDeps,
+  type PrNodeGithubOps,
+  type PrNodeStore,
+  type PrSourceDescriptor,
+  type PrCreateCallInput,
+  type PrCreateCallResult,
+  type PrMergeCallInput,
+  type PrMergeCallResult,
+  type PrRespondCallInput,
+  type PrRespondCallResult,
+} from "./pr-nodes.js";
 export {
   WorkflowGraphTaskRunner,
   type WorkflowGraphRunDisposition,