fix: address review findings for self-update (#15)

CLAUDE.md

@@ -58,14 +58,12 @@ DATABASE_URL=postgres://... go test -tags integration ./internal/master/store/
   the agent's own container whose source equals its destination.** Mounts are read per
   job (`ContainerInspect(SELF_ID)`) and checked after symlink resolution. This replaces
   the old `STACKS_ROOT`.
-- **Self-update**: an agent's own Compose service is an ordinary target. Pull runs in the
-  old agent; `up -d` runs in a helper container made from the agent's image (same mounts),
-  labelled `se.shcizo.package-updater.self-update.*`. The new agent finds the helper at
-  startup (`updater.ResumeSelfUpdates`, before runner and connection start) and reports
-  the result under the old job ID. The old agent records no result once SIGTERM cancels
-  its context. Self jobs (`JobStart.Self`) run after every other queued job. An agent's
-  own Swarm service is refused (`ReasonSelfSwarm`). Needs protocol 4. The master updates
-  itself with no special code: the agent on its host resends the result.
+- **Self-update** (an agent's own Compose service): only `up -d` runs in the helper
+  container; the new agent resumes the job (`updater.ResumeSelfUpdates`) before the runner
+  and connection start; the old agent records no result once SIGTERM cancels it. Jobs for
+  the agent's own service run last on the agent and the master starts them last in a
+  batch. An agent's own Swarm service is refused (`ReasonSelfSwarm`). Protocol 4 is needed,
+  also for any service with the `AGENT_IMAGE` repo.
 - **Unknown message types are logged and ignored; malformed messages close the connection.**
 - **Reconcile before routable**: the hub fails a reconnecting agent's jobs it does not
   list in `known_jobs`, and only then accepts jobs for that connection.
@@ -131,8 +129,9 @@ DATABASE_URL=postgres://... go test -tags integration ./internal/master/store/
 - **`protocol.Service.ConfigFiles` is the raw comma-joined label string**, so `Service`
   stays comparable with `==` (`inventory.Diff` depends on it).
 - **Helper containers carry `se.shcizo.package-updater.self-update.*` labels** and are
-  not inventoried (the collector skips any container with the `.job` label). The labels
-  are constants in `internal/protocol`.
+  not inventoried (the collector skips any container with the `.job` label). The label
+  strings are constants in `internal/agent/updater/selfupdate.go`, and the `.job` one is
+  repeated in `internal/agent/inventory`.
 - **Swarm "succeeded" means the daemon accepted the new spec**, not that tasks converged.
 - **Swarm updates need manager-node API access** for the agent doing the update.
 - **The image's default entrypoint is `updater-agent`.** The master overrides it with
@@ -145,7 +144,7 @@ DATABASE_URL=postgres://... go test -tags integration ./internal/master/store/
   knows about; only the waiting stops. The master never fails or cancels a timed-out job.
 - **A webhook job's target is `<image_repo>:<request tag>`**, built from the service's own
   repo, never from the request's image string. With an explicit target the "follows no tag"
-  refusal does not apply; removed, built, container and Swarm self still do.
+  refusal does not apply; removed, built, container and an agent's own Swarm service still do.
 - **`available_*` is cleared by `FinishJob`** on success only when the job's `to_ref`
   (as created, without `@digest`) equals `available_ref`, `available_at <= job.created_at`
   (that version is now running) and, when the job's `to_ref` carries a digest, that digest