chore(ci): tidy CI/dev infra — drop local-ci, lift migration rule, scope by galaxy.stack label

Five connected cleanups across the dev/CI infrastructure:

1. Drop tools/local-ci/. The standalone Gitea + act_runner stack was
   the legacy "offline workflow validator"; the per-stage CI gate now
   runs on gitea.lan and the directory was only retained as a
   fallback. Removing it leaves no operational dependency: backend,
   gateway, and game code have no references; documentation that
   pointed at it (CLAUDE.md, docs/ARCHITECTURE.md, ui/docs/testing.md,
   tools/dev-deploy/README.md, tools/local-dev/README.md) is updated
   in this same change. Historical "Verified on local-ci run N"
   markers in ui/PLAN.md are preserved unchanged.

2. Lift the pre-production single-migration rule. The rule forced
   every schema delta into 00001_init.sql and required a manual
   make clean-data wipe on every backward-incompatible change in
   tools/dev-deploy/. Future schema deltas now land as additive
   sequence-numbered files (00002_*.sql, …) that goose applies
   automatically on backend startup; 00001_init.sql becomes an
   immutable baseline. Authoring conventions live in
   backend/internal/postgres/migrations/README.md. The chain may be
   squashed back into a fresh 00001 as a deliberate one-time
   operation before the first production deployment.

3. Document the deployment cadence. The dev environment is
   single-tenant: pushes to feature/* run the test workflows
   (go-unit, ui-test, integration) only; dev-deploy.yaml fires on
   push to development. A workflow_dispatch override on
   dev-deploy.yaml lets a developer preview a feature branch on the
   shared dev environment before merge; the next merge into
   development overwrites the manual deploy idempotently.

4. Scope compose-managed resources by an explicit
   galaxy.stack=<local-dev|dev-deploy> label. Both compose files
   stamp the label on every service, network, and named volume.
   Makefiles in tools/local-dev/ and tools/dev-deploy/ filter their
   engine-cleanup operations by (stack-label AND engine OCI title)
   so they never touch unrelated workloads on the same daemon.
   dev-deploy.yaml gains a pre-`compose up` step that reaps stale
   exited/dead containers under the dev-deploy stack label.

5. Backend now stamps the same galaxy.stack=<value> label on every
   engine container it spawns, sourced from a new BACKEND_STACK_LABEL
   env var (empty → label not applied; legacy-safe). Both compose
   files set it to their stack name (local-dev / dev-deploy). The
   contract is recorded in docs/ARCHITECTURE.md under
   "Container labels". A package-level test in
   backend/internal/runtime exercises both the label-present and
   label-absent paths.

No tests intentionally regressed: go test ./backend/internal/{config,
runtime,dockerclient} is green, both compose files validate cleanly,
and the backend, gateway, and game modules all build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/config/config.go

@@ -91,6 +91,7 @@ const (
 	envRuntimeContainerPIDsLimit = "BACKEND_RUNTIME_CONTAINER_PIDS_LIMIT"
 	envRuntimeContainerStateMount = "BACKEND_RUNTIME_CONTAINER_STATE_MOUNT"
 	envRuntimeStopGracePeriod = "BACKEND_RUNTIME_STOP_GRACE_PERIOD"
+	envRuntimeStackLabel = "BACKEND_STACK_LABEL"
 
 	envNotificationAdminEmail = "BACKEND_NOTIFICATION_ADMIN_EMAIL"
 	envNotificationWorkerInterval = "BACKEND_NOTIFICATION_WORKER_INTERVAL"
@@ -409,6 +410,14 @@ type RuntimeConfig struct {
 	// StopGracePeriod is the docker stop SIGTERM-to-SIGKILL grace period
 	// applied during stop / cancel / restart / patch.
 	StopGracePeriod time.Duration
+
+	// StackLabel is the optional value backend stamps as
+	// `galaxy.stack=<value>` on every engine container it spawns. It
+	// lets host-side tooling (Makefile, CI workflows) scope cleanup
+	// operations to a single dev stack without touching unrelated
+	// workloads on the same Docker daemon. When empty, the label is
+	// not applied.
+	StackLabel string
 }
 
 // DiplomailConfig bounds the diplomatic-mail subsystem. Both limits
@@ -705,6 +714,7 @@ func LoadFromEnv() (Config, error) {
 	if cfg.Runtime.StopGracePeriod, err = loadDuration(envRuntimeStopGracePeriod, cfg.Runtime.StopGracePeriod); err != nil {
 		return Config{}, err
 	}
+	cfg.Runtime.StackLabel = strings.TrimSpace(loadString(envRuntimeStackLabel, cfg.Runtime.StackLabel))
 
 	cfg.Notification.AdminEmail = loadString(envNotificationAdminEmail, cfg.Notification.AdminEmail)
 	if cfg.Notification.WorkerInterval, err = loadDuration(envNotificationWorkerInterval, cfg.Notification.WorkerInterval); err != nil {