feat(FN-837): add workflow step phase support with pre-merge and post-merge execution

- Add phase field to workflow step definitions (pre-merge vs post-merge) with persistence and API
- Execute pre-merge steps in executor before merge; post-merge steps in merger after successful merge
- Pre-merge failures block merge and keep task in in-review; post-merge failures are logged only
- Expose phase controls and phase-aware results in the dashboard UI
- Add changeset for the published @gsxdsm/fusion package

packages/core/src/task-merge.test.ts

@@ -0,0 +1,183 @@
+import { describe, it, expect } from "vitest";
+import type { StepStatus } from "./types.js";
+import { getTaskMergeBlocker, isTaskReadyForMerge } from "./task-merge.js";
+
+const baseTask = {
+  column: "in-review" as const,
+  paused: false,
+  status: undefined as string | undefined,
+  error: undefined as string | undefined,
+  steps: [] as Array<{ name: string; status: StepStatus }>,
+  workflowStepResults: undefined as any,
+};
+
+describe("getTaskMergeBlocker", () => {
+  it("returns undefined for a clean task in review", () => {
+    expect(getTaskMergeBlocker(baseTask)).toBeUndefined();
+  });
+
+  it("returns reason when task is not in review", () => {
+    expect(getTaskMergeBlocker({ ...baseTask, column: "todo" }))
+      .toContain("must be in 'in-review'");
+  });
+
+  it("returns reason when task is paused", () => {
+    expect(getTaskMergeBlocker({ ...baseTask, paused: true }))
+      .toBe("task is paused");
+  });
+
+  it("returns reason when task has failed status", () => {
+    expect(getTaskMergeBlocker({ ...baseTask, status: "failed" }))
+      .toContain("failed");
+  });
+
+  it("returns reason when task has incomplete steps", () => {
+    expect(getTaskMergeBlocker({
+      ...baseTask,
+      steps: [{ name: "Step 1", status: "in-progress" }],
+    })).toBe("task has incomplete steps");
+  });
+
+  // ── Workflow Step Phase Awareness ──────────────────────────────────────
+
+  it("blocks merge when pre-merge workflow step has failed", () => {
+    const result = getTaskMergeBlocker({
+      ...baseTask,
+      workflowStepResults: [{
+        workflowStepId: "WS-001",
+        workflowStepName: "Pre-merge Check",
+        phase: "pre-merge",
+        status: "failed",
+        output: "Check failed",
+      }],
+    });
+    expect(result).toContain("pre-merge workflow steps");
+  });
+
+  it("blocks merge when legacy workflow step (no phase) has failed", () => {
+    const result = getTaskMergeBlocker({
+      ...baseTask,
+      workflowStepResults: [{
+        workflowStepId: "WS-001",
+        workflowStepName: "Legacy Check",
+        // phase is undefined → treated as pre-merge
+        status: "failed",
+        output: "Check failed",
+      }],
+    });
+    expect(result).toContain("pre-merge workflow steps");
+  });
+
+  it("does NOT block merge when only post-merge workflow step has failed", () => {
+    const result = getTaskMergeBlocker({
+      ...baseTask,
+      workflowStepResults: [{
+        workflowStepId: "WS-001",
+        workflowStepName: "Post-merge Notify",
+        phase: "post-merge",
+        status: "failed",
+        output: "Notification failed",
+      }],
+    });
+    expect(result).toBeUndefined();
+  });
+
+  it("does NOT block merge when pre-merge passed and post-merge failed", () => {
+    const result = getTaskMergeBlocker({
+      ...baseTask,
+      workflowStepResults: [
+        {
+          workflowStepId: "WS-001",
+          workflowStepName: "Pre-merge Check",
+          phase: "pre-merge",
+          status: "passed",
+        },
+        {
+          workflowStepId: "WS-002",
+          workflowStepName: "Post-merge Notify",
+          phase: "post-merge",
+          status: "failed",
+          output: "Failed",
+        },
+      ],
+    });
+    expect(result).toBeUndefined();
+  });
+
+  it("blocks merge when pre-merge step is still pending", () => {
+    const result = getTaskMergeBlocker({
+      ...baseTask,
+      workflowStepResults: [{
+        workflowStepId: "WS-001",
+        workflowStepName: "Pre-merge Check",
+        phase: "pre-merge",
+        status: "pending",
+      }],
+    });
+    expect(result).toContain("pre-merge workflow steps");
+  });
+
+  it("does NOT block merge when only post-merge step is pending", () => {
+    const result = getTaskMergeBlocker({
+      ...baseTask,
+      workflowStepResults: [{
+        workflowStepId: "WS-001",
+        workflowStepName: "Post-merge Notify",
+        phase: "post-merge",
+        status: "pending",
+      }],
+    });
+    expect(result).toBeUndefined();
+  });
+
+  it("allows merge when all pre-merge steps passed regardless of post-merge status", () => {
+    const result = getTaskMergeBlocker({
+      ...baseTask,
+      workflowStepResults: [
+        {
+          workflowStepId: "WS-001",
+          workflowStepName: "Pre-merge Check",
+          phase: "pre-merge",
+          status: "passed",
+        },
+        {
+          workflowStepId: "WS-002",
+          workflowStepName: "Post-merge Verify",
+          phase: "post-merge",
+          status: "skipped",
+        },
+      ],
+    });
+    expect(result).toBeUndefined();
+  });
+});
+
+describe("isTaskReadyForMerge", () => {
+  it("returns true for a clean task in review", () => {
+    expect(isTaskReadyForMerge(baseTask)).toBe(true);
+  });
+
+  it("returns false when pre-merge step failed", () => {
+    expect(isTaskReadyForMerge({
+      ...baseTask,
+      workflowStepResults: [{
+        workflowStepId: "WS-001",
+        workflowStepName: "Check",
+        phase: "pre-merge",
+        status: "failed",
+      }],
+    })).toBe(false);
+  });
+
+  it("returns true when only post-merge step failed", () => {
+    expect(isTaskReadyForMerge({
+      ...baseTask,
+      workflowStepResults: [{
+        workflowStepId: "WS-001",
+        workflowStepName: "Notify",
+        phase: "post-merge",
+        status: "failed",
+      }],
+    })).toBe(true);
+  });
+});