docs: index agent-activity-contract.md in README

Add missing index entry for docs/agent-activity-contract.md in the
Architecture & Development section. The doc was a genuine orphan —
a canonical wire/cursor/retention contract for GET /api/agent-activity
referenced in AGENTS.md but not discoverable from the docs index.

docs/README.md

@@ -58,6 +58,7 @@ Planner oversight (FN-7508 → FN-7583) is fully documented in Settings Referenc
 | [Architecture](./architecture.md) | System architecture, package layout, storage model, and engine execution flow |
 | [Secrets Store (`SecretsStore`)](./architecture.md#secrets-store-secretsstore) | Core encrypted secret subsystem overview: scopes, AES-256-GCM at-rest model, policy semantics, and public store API surface |
 | [Dashboard Real-Time](./dashboard-realtime.md) | Canonical event-stream architecture contract (shared `/api/events` bus + dedicated stream boundaries), with project/node scoping, reconnect/cleanup behavior, and realtime pitfalls |
+| [Agent Activity Contract](./agent-activity-contract.md) | Canonical wire, cursor, and retention contract for the project-scoped `GET /api/agent-activity` durable activity-history API |
 | [Storage](./storage.md) | PostgreSQL runtime storage, archive, migration compatibility, and file-backed payloads |
 | [DAG Architecture Deliverables](./dag/) | Milestone A DAG architecture documents plus Milestone B prototype scaffold docs (schema migration plan, DagCoordinator design, implementation checklist) |
 | [Dev Server Module Audit](./dev-server-modules.md) | Analysis of parallel dashboard dev-server module families, production wiring, and consolidation guidance |