fix(FN-branch-group): promotion lock, PR repair, audit on failure, typed sync block

Review residuals #3/#4/#6/#10: per-group in-process promotion lock (concurrent
route+auto promotion could double-create PRs), finalized-but-PR-less groups can
be repaired by re-promotion without re-merging, auto-promotion failures emit
merge:branch-group-promotion-failed instead of silent swallow, exported
reconcileBranchGroupPr for out-of-band merged reconciliation, and the merger
sync block drops its (store as any) casts (TaskStore already carries the
methods).

packages/engine/src/index.ts

@@ -61,10 +61,13 @@ export {
   evaluateBranchGroupPromotion,
   evaluateBranchGroupCompletion,
   promoteBranchGroup,
+  reconcileBranchGroupPr,
   type BranchGroupMergeRouting,
   type BranchGroupPromotionDecision,
   type BranchGroupCompletionStatus,
   type BranchGroupPromotionResult,
+  type PromoteBranchGroupInput,
+  type ReconcileBranchGroupPrResult,
   type CreateGroupPrFn,
   type SyncGroupPrFn,
   type CloseGroupPrFn,