feat(FN-3217): honor resolved merge target across completion flows

- Add merge target branch resolver contract to core types and exports
- Update task lifecycle completion path to resolve and propagate merge target branch
- Use resolved merge target in merger execution instead of stale/default branch assumptions
- Expand core and CLI tests for merge-target resolution behavior and add changeset for @runfusion/fusion

Fusion-Task-Id: FN-3217

packages/core/src/task-merge.ts

@@ -1,5 +1,38 @@
 import type { Task, WorkflowStepResult } from "./types.js";
 
+export interface MergeTargetResolution {
+  branch: string;
+  source: "task-base-branch" | "task-branch-context" | "project-default" | "legacy-main";
+}
+
+export interface MergeTargetResolverOptions {
+  projectDefaultBranch?: string;
+  legacyFallbackBranch?: string;
+}
+
+export function resolveTaskMergeTarget(
+  task: Pick<Task, "baseBranch" | "branchContext">,
+  options: MergeTargetResolverOptions = {},
+): MergeTargetResolution {
+  const configuredBase = task.baseBranch?.trim();
+  if (configuredBase) {
+    return { branch: configuredBase, source: "task-base-branch" };
+  }
+
+  const inheritedBase = task.branchContext?.inheritedBaseBranch?.trim();
+  if (inheritedBase) {
+    return { branch: inheritedBase, source: "task-branch-context" };
+  }
+
+  const projectDefault = options.projectDefaultBranch?.trim();
+  if (projectDefault) {
+    return { branch: projectDefault, source: "project-default" };
+  }
+
+  const legacyFallback = options.legacyFallbackBranch?.trim() || "main";
+  return { branch: legacyFallback, source: "legacy-main" };
+}
+
 const BLOCKING_TASK_STATUSES = new Set([
   "failed",
   // ── User-attention / awaiting-handoff states ─────────────────────────