FN-5830: re-land shared branch promotion gate and API

Reintroduce shared-branch-group completion gating and promotion endpoints with reliability coverage.

- add GroupMergeCoordinator promotion gate behavior and shared branch group promotion logic updates
- expose promotion flow wiring through engine index and project engine APIs
- expand coordinator tests and add reliability interaction coverage for branch-group promotion
- document the new reliability backstop and add a changeset for @runfusion/fusion

Files changed:
 .changeset/fn-5830-branch-group-promotion.md       |   5 +
 AGENTS.md                                          |   1 +
 docs/architecture.md                               |   3 +-
 .../src/__tests__/group-merge-coordinator.test.ts  | 170 ++++++++++++++++++-
 .../branch-group-promotion.test.ts                 | 166 +++++++++++++++++++
 packages/engine/src/group-merge-coordinator.ts     | 183 ++++++++++++++++++++-
 packages/engine/src/index.ts                       |   4 +
 packages/engine/src/project-engine.ts              |  37 +++++
 8 files changed, 566 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-5830

Fusion-Task-Lineage: e9823f51-df0e-4da8-8f51-58dfefdc293d

packages/engine/src/index.ts

@@ -59,8 +59,12 @@ export {
 export {
   resolveBranchGroupMergeRouting,
   evaluateBranchGroupPromotion,
+  evaluateBranchGroupCompletion,
+  promoteBranchGroup,
   type BranchGroupMergeRouting,
   type BranchGroupPromotionDecision,
+  type BranchGroupCompletionStatus,
+  type BranchGroupPromotionResult,
 } from "./group-merge-coordinator.js";
 export {
   resolveMergeIntegrationRoot,