Document the discovery API in docs/api.md

Full client-facing reference: auth, all routes with schemas and curl
examples, selection/cooldown semantics, caveats. README and
architecture.md link to it.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/architecture.md

@@ -213,6 +213,8 @@ Kubernetes interaction is reading Proxies from the manager's cache. The
 server is a non-leader-elected Runnable (all replicas would serve, but the
 deployment ships `replicas: 1` because lease state is per-process — an
 operator restart drops all leases and cooldowns, a documented caveat).
+Client-facing reference with request/response schemas and curl examples:
+[api.md](api.md).
 
 ```text
 crawler client