docs(changelog): runner fleet execution-plane split

changelog.mdx

@@ -18,6 +18,26 @@ export const STAGE_RATE_M65 = "$0.10";
   usezombie is in **stealth-mode testing** and pre-production. APIs and agent behavior may change between releases without long deprecation windows. Email [usezombie@agentmail.to](mailto:usezombie@agentmail.to) if you want a hand calibrating a zombie or to join as a design partner.
 </Tip>
 
+<Update label="May 27, 2026" tags={["What's new", "Internal"]}>
+  ## Execution moves to a host-resident runner fleet
+
+  Behind an unchanged user surface, an event's agent now runs in a separate `zombie-runner` daemon instead of inside the API server. `zombied` became the control plane — it owns Postgres, Redis, and the Vault, and hands work to runners over an authenticated HTTPS protocol — while the runner is the execution plane that runs each event in an isolated sandbox holding no datastore credentials. Steering, webhooks, cron, the live event tail, and history behave exactly as before; what changed is where the work runs.
+
+  ## What's new
+
+  - **Control plane / execution plane split.** A runner leases an event, runs it, and reports the result; `zombied` does the durable writes. Work can run on hosts that never see a database credential.
+  - **Lease-based ownership.** Each lease carries a deadline. A runner that dies mid-event has its work reclaimed and re-run by another runner; a late report from the dead runner is rejected, so state is never double-written.
+  - **Sandbox is mandatory.** Every event runs in a sandbox; a sandbox that fails to start fails closed rather than running unprotected.
+
+  <Note>
+    Host-resident runners are not enabled in production yet — this release lands the architecture; turning them on follows in a later update.
+  </Note>
+
+  <Note>
+    One known limit: an agent that runs longer than the 30-second lease window is reclaimed and re-run, so long single events wait on a follow-up that adds lease renewal.
+  </Note>
+</Update>
+
 <Update label="May 22, 2026" tags={["What's new", "CLI", "Security"]}>
   ## `zombiectl login` — verification-code device flow + non-interactive token auth