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/pipeline.ts

@@ -0,0 +1,58 @@
+/**
+ * INTERIM ORCHESTRATOR. FN-3779 replaces this with cadence-registry + cron-sentinel wiring;
+ * FN-3780 replaces the aggregate dependency with the real aggregation layer.
+ * Keep the `ReportsPipelineDependencies` shape stable so both can plug in without callsite churn.
+ */
+import type { ReportsAggregator } from "./aggregation.js";
+import type { ReportsCadence } from "./cadence.js";
+import type { ReportRunRecord, ReportsRunsStore } from "./runs-store.js";
+
+export interface ReportsPipelineDependencies {
+  runsStore: ReportsRunsStore;
+  aggregate: ReportsAggregator;
+}
+
+export interface StartPipelineInput {
+  runId: string;
+  cadence: ReportsCadence;
+  settings: Record<string, unknown>;
+  now?: Date;
+}
+
+export async function startReportsPipeline(
+  input: StartPipelineInput,
+  deps: ReportsPipelineDependencies,
+): Promise<ReportRunRecord> {
+  const nowIso = (input.now ?? new Date()).toISOString();
+
+  const created = await deps.runsStore.create({
+    id: input.runId,
+    cadence: input.cadence,
+    status: "queued",
+    createdAt: nowIso,
+    updatedAt: nowIso,
+  });
+
+  await deps.runsStore.update(input.runId, { status: "running", updatedAt: nowIso });
+
+  try {
+    await deps.aggregate({
+      runId: input.runId,
+      cadence: input.cadence,
+      settings: input.settings,
+    });
+
+    const updatedAt = new Date().toISOString();
+    const reviewed = await deps.runsStore.update(input.runId, { status: "review", updatedAt });
+    return reviewed ?? { ...created, status: "review", updatedAt };
+  } catch (error) {
+    const updatedAt = new Date().toISOString();
+    const message = error instanceof Error ? error.message : String(error);
+    const failed = await deps.runsStore.update(input.runId, {
+      status: "failed",
+      error: message,
+      updatedAt,
+    });
+    return failed ?? { ...created, status: "failed", error: message, updatedAt };
+  }
+}