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/aggregation.ts

@@ -0,0 +1,23 @@
+/**
+ * INTERIM SEAM — replaced by FN-3780's real aggregation orchestrator.
+ * Keep the type surface stable so FN-3780 can swap the implementation without renaming exports.
+ */
+import type { ReportsCadence } from "./cadence.js";
+
+export interface ReportAggregationInput {
+  runId: string;
+  cadence: ReportsCadence;
+  settings: Record<string, unknown>;
+}
+
+export interface ReportAggregationOutput {
+  summary: string;
+  sections: Array<{ id: string; title: string; body: string }>;
+}
+
+export type ReportsAggregator = (input: ReportAggregationInput) => Promise<ReportAggregationOutput>;
+
+export const aggregateReportData: ReportsAggregator = async ({ cadence }) => ({
+  summary: `Aggregation scaffold not yet implemented for ${cadence} reports.`,
+  sections: [],
+});