docs: describe one-instance-per-server topology and why MODE is exclusive

README.md

@@ -46,6 +46,28 @@ Two things to get right when running in Swarm mode:
   mounted. This is a deployment concern the service cannot detect or work
   around.
 
+## Deploying across multiple servers
+
+An instance can only reach the Docker daemon it is configured against. It cannot
+update stacks on other machines. The deployment model follows from that:
+
+**One instance per server.** Each server runs its own package-updater against its
+own local Docker socket, with `MODE` set to whatever that machine runs:
+
+| Server | `MODE` | Updates |
+|---|---|---|
+| Swarm manager node | `swarm` | All Swarm services in the cluster |
+| Standalone Compose host | `compose` | Compose stacks on that host |
+
+**CI fans out.** The [Gitea composite action](gitea-action/README.md) takes a list of
+endpoints and posts `/update` to every instance, so one workflow run reaches the
+whole fleet. Each instance answers for its own machine; there is no aggregated
+cross-server response and no instance coordinates any other.
+
+This is why `MODE` is exclusive rather than a mode that handles both at once: an
+instance that could do both would still only reach one daemon, so the extra
+generality buys nothing.
+
 ## Quick start
 
 1. Build and push the image (e.g. via your own CI).
@@ -68,7 +90,7 @@ All via environment variables.
 | `LOG_LEVEL` | no | `info` | `debug` / `info` / `warn` / `error`. |
 | `UPDATE_TIMEOUT` | no | `5m` | Per-job timeout (Go duration). |
 | `OPT_IN_LABEL` | no | `se.shcizo.auto-update` | Label name to check; value must equal `"true"`. |
-| `MODE` | no | `compose` | `compose` or `swarm`. Selects the update mechanism for the whole deployment; not mixed per-request. |
+| `MODE` | no | `compose` | `compose` or `swarm`. Which paradigm *this instance's* Docker daemon runs. See [Deploying across multiple servers](#deploying-across-multiple-servers). |
 
 ## Endpoints
 
@@ -100,7 +122,8 @@ These are tracked in the spec's section 2 and section 15 as deliberate out-of-sc
 
 - **Self-update wiring**: `internal/selfupdate.Wrapped` exists and is unit-tested but is not wired into the live queue. The HTTP response flush ordering for self-replacement is a future enhancement; for now, expect to manually rerun `docker compose up -d` on the host if pushing a new image of `package-updater` itself causes a mid-response interruption.
 - **No rollback**: Compose's "keep old container if new fails to start" is the only safety net.
-- **Single host only in Compose mode**. Swarm mode (`MODE=swarm`) is the
-  multi-node path, but only from a manager node's point of view — the updater
-  itself still needs manager API access (see "Swarm mode" above).
+- **One instance reaches one daemon.** An instance never updates another server;
+  fleets run one instance per server with CI fanning out to all of them (see
+  "Deploying across multiple servers"). Swarm mode is the exception in that a
+  single manager-node instance covers the whole cluster.
 - **No per-repo API keys**: a single shared bearer token is used.