swarm-controller: record builder runs with nix's tail and link the status to them

swarm-controller/README.md

@@ -79,6 +79,12 @@ The swarm's control plane. The swarm UI and `swarmctl` are its clients.
   leaves the group drops out of the list. Every agent reads the key into its
   system prompt →
   [`docs/turn-loop/claude-invocation.md`](../docs/turn-loop/claude-invocation.md).
+- **Builder runs** — the swarm builder's newest 100 finished builds of agent
+  config PRs, with `nix`'s tail for a failed one, one document in the
+  `builder-runs` KV bucket, which this daemon alone writes
+  (`GET /api/builder/runs`, `GET /api/builder/runs/{id}`, the link on each
+  `swarm-builder` commit status) →
+  [`docs/swarm/builder.md`](../docs/swarm/builder.md#build-records).
 - **Relays** — each agent's terminal and turn-state header as SSE, agent
   icons, the cross-repo issue report, and the UI's quick links.
 - **Alerts** — the swarm Grafana's current alerts (`GET /api/alerts`), read