FN-6030: fix workflow terminal notifications

Ensure merged workflow tasks still emit the expected terminal notifications.

- emit task:merged when PR-driven workflow merges move tasks to done
- send merged notifications for merge-backed done transitions and suppress duplicates
- add regression coverage, a published changeset, and reconcile overlapping docs/test updates

Files changed:
 .../fn-6030-workflow-terminal-notifications.md     |   5 +
 docs/storage.md                                    |   8 +-
 .../core/src/__tests__/builtin-workflows.test.ts   |   1 -
 .../__tests__/store-pr-merged-transition.test.ts   |  14 ++-
 .../src/__tests__/workflow-ir-resolver.test.ts     |   1 -
 packages/core/src/store.ts                         |  14 ++-
 .../settings/sections/ProjectModelsSection.tsx     |   3 +-
 .../engine/src/__tests__/merger-post-merge.test.ts |   9 +-
 .../src/__tests__/notification-service.test.ts     | 129 ++++++++++++++++++++-
 .../__tests__/workflow-graph-task-runner.test.ts   |  57 +++++++++
 .../src/notification/notification-service.ts       |  36 ++++--
 11 files changed, 257 insertions(+), 20 deletions(-)

Fusion-Task-Id: FN-6030

Fusion-Task-Lineage: a991f765-0986-4541-ab38-ff476cf88d16

packages/core/src/store.ts

@@ -11809,13 +11809,25 @@ ${TASK_UPSERT_SQL_ASSIGNMENTS}
       return { moved: false, skipped: "wrong-column" };
     }
 
-    await this.moveTask(taskId, "done", {
+    const movedTask = await this.moveTask(taskId, "done", {
       moveSource: "engine",
       preserveProgress: true,
       preserveWorktree: true,
       skipMergeBlocker: true,
     });
 
+    this.emit("task:merged", {
+      task: movedTask,
+      branch: movedTask.branch ?? movedTask.prInfo?.headBranch ?? freshTask.branch ?? freshTask.prInfo?.headBranch ?? "",
+      merged: true,
+      worktreeRemoved: false,
+      branchDeleted: false,
+      mergeConfirmed: movedTask.mergeDetails?.mergeConfirmed ?? freshTask.mergeDetails?.mergeConfirmed,
+      mergedAt: movedTask.mergeDetails?.mergedAt ?? freshTask.mergeDetails?.mergedAt,
+      mergeTargetBranch: movedTask.mergeDetails?.mergeTargetBranch ?? freshTask.mergeDetails?.mergeTargetBranch,
+      mergeTargetSource: movedTask.mergeDetails?.mergeTargetSource ?? freshTask.mergeDetails?.mergeTargetSource,
+    } satisfies MergeResult);
+
     if (ctx?.agentId && ctx?.runId) {
       this.recordRunAuditEvent({
         taskId,