feat(pr): top-level bounded rework + auto-merge gate + legacy-bypass pin (U6)

Generalizes the foreach-only bounded-rework mechanism to the top-level
graph walk so the PR review loop (await-review -> pr-respond -> rework
back) is a legal bounded cycle: a kind:rework back-edge to a stacked node
returns a ReworkSignal the loop-head re-runs up to maxReworkCycles, then
routes outcome:rework-exhausted. Non-rework cycles still throw 'Cycle
detected' (safety preserved); foreach rework unchanged (shared core
constants). Adds createAutoMergeGateHandler (live entity re-fetch +
isPrEntityAutoMergeReady -> auto-on/auto-off). Pins R14: graph-executed
PR tasks merge through pr-merge, never the legacy queue. 122 graph tests
green.

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

packages/engine/src/index.ts

@@ -48,6 +48,7 @@ export {
 } from "./workflow-node-handlers.js";
 export {
   createPrNodeHandlers,
+  createAutoMergeGateHandler,
   buildPrNodeDeps,
   type PrNodeDeps,
   type PrNodeGithubOps,