feat(FN-3860): add cadence resolution seam (+7 more)

Commits merged:
- feat(FN-3860): complete Step 8 — documentation and delivery
- test(FN-3860): complete Step 7 — add scaffold seam coverage
- feat(FN-3860): complete Step 6 — re-export scaffold seams
- feat(FN-3860): complete Step 5 — add pipeline orchestrator seam
- feat(FN-3860): complete Step 4 — add in-memory runs store seam
- feat(FN-3860): complete Step 3 — add aggregation seam
- feat(FN-3860): complete Step 2 — add cadence resolution seam
- feat(FN-3846): short-circuit phantom merges for already-landed branches

Files changed:
.changeset/fn-3860-reports-scaffold-files.md       |  5 ++
 plugins/fusion-plugin-reports/README.md            |  9 +++
 .../src/__tests__/scaffold.test.ts                 | 60 +++++++++++++++++++
 plugins/fusion-plugin-reports/src/aggregation.ts   | 23 ++++++++
 plugins/fusion-plugin-reports/src/cadence.ts       | 23 ++++++++
 plugins/fusion-plugin-reports/src/index.ts         |  4 ++
 plugins/fusion-plugin-reports/src/pipeline.ts      | 58 ++++++++++++++++++
 plugins/fusion-plugin-reports/src/runs-store.ts    | 69 ++++++++++++++++++++++
 8 files changed, 251 insertions(+)

Fusion-Task-Id: FN-3860

plugins/fusion-plugin-reports/src/__tests__/scaffold.test.ts

@@ -0,0 +1,60 @@
+import { describe, expect, it } from "vitest";
+import { aggregateReportData } from "../aggregation.js";
+import { resolveEnabledCadences } from "../cadence.js";
+import { startReportsPipeline } from "../pipeline.js";
+import { createInMemoryReportsRunsStore } from "../runs-store.js";
+
+describe("reports scaffold seams", () => {
+  it("resolves daily and weekly cadence by default in UTC", () => {
+    expect(resolveEnabledCadences({})).toEqual([
+      { cadence: "daily", timezone: "UTC" },
+      { cadence: "weekly", timezone: "UTC" },
+    ]);
+  });
+
+  it("resolves weekly-only cadence with configured timezone", () => {
+    expect(
+      resolveEnabledCadences({ dailyEnabled: false, weeklyEnabled: true, timezone: "America/Los_Angeles" }),
+    ).toEqual([{ cadence: "weekly", timezone: "America/Los_Angeles" }]);
+  });
+
+  it("returns empty sections from scaffold aggregator", async () => {
+    const output = await aggregateReportData({ runId: "run-1", cadence: "daily", settings: {} });
+    expect(output.sections).toEqual([]);
+  });
+
+  it("runs pipeline to review status on happy path", async () => {
+    const runsStore = createInMemoryReportsRunsStore();
+    const result = await startReportsPipeline(
+      { runId: "run-1", cadence: "daily", settings: {} },
+      { runsStore, aggregate: aggregateReportData },
+    );
+
+    expect(result.status).toBe("review");
+    const stored = await runsStore.get("run-1");
+    expect(stored).toEqual(result);
+    expect(stored?.cadence).toBe("daily");
+  });
+
+  it("marks pipeline run failed when aggregation throws and does not rethrow", async () => {
+    const runsStore = createInMemoryReportsRunsStore();
+    const result = await startReportsPipeline(
+      { runId: "run-2", cadence: "weekly", settings: {} },
+      {
+        runsStore,
+        aggregate: async () => {
+          throw new Error("boom");
+        },
+      },
+    );
+
+    expect(result.status).toBe("failed");
+    expect(result.error).toBe("boom");
+    await expect(runsStore.get("run-2")).resolves.toEqual(result);
+  });
+
+  it("returns undefined when updating unknown run", async () => {
+    const runsStore = createInMemoryReportsRunsStore();
+    await expect(runsStore.update("missing", { status: "failed" })).resolves.toBeUndefined();
+  });
+});