feat(FN-5438): add manual merge blocker bypass mode for queued tasks

Adds a manual merge blocker mode (FN-5438) that prevents automatic merging and provides a bypass mechanism to resume, wired through the merger, project engine, and task workflow API routes. Includes tests across core, engine route registration, and project engine layers, plus a changeset and documen

Fusion-Task-Id: FN-5438

packages/engine/src/project-engine.ts

@@ -1531,6 +1531,7 @@ export class ProjectEngine {
               this.activeMergeTaskId = taskId;
               this.mergeAbortController = new AbortController();
               return aiMergeTask(store, cwd, taskId, {
+                manual: !!manualResolver,
                 pool,
                 usageLimitPauser,
                 agentStore,