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>

tools/local-dev/Makefile

@@ -5,9 +5,16 @@
 COMPOSE := docker compose
 REPO_ROOT := $(realpath $(CURDIR)/../..)
 ENGINE_IMAGE := galaxy-engine:local-dev
-# Label set by the engine `Dockerfile` runtime stage; used to find
-# engine containers spawned by backend's runtime that fall outside
-# `docker compose down`'s scope.
+# Engine containers spawned by backend's runtime fall outside the
+# compose project. We identify them by two labels:
+#   STACK_LABEL  — backend stamps this on every engine it spawns from
+#                  this stack (see BACKEND_STACK_LABEL env in the
+#                  compose file);
+#   ENGINE_LABEL — image-level OCI title baked into the engine
+#                  Dockerfile.
+# Both filters together select exactly this stack's engine containers
+# and never compose-managed services or unrelated workloads.
+STACK_LABEL := galaxy.stack=local-dev
 ENGINE_LABEL := org.opencontainers.image.title=galaxy-game-engine
 
 help:
@@ -65,9 +72,11 @@ clean: stop-engines
 # cascade the game to `cancelled`. We only remove them as part of
 # `clean`, where the whole DB is wiped anyway.
 stop-engines:
-	@ids=$$(docker ps -aq --filter label=$(ENGINE_LABEL)); \
+	@ids=$$(docker ps -aq \
+		--filter "label=$(STACK_LABEL)" \
+		--filter "label=$(ENGINE_LABEL)"); \
 	if [ -n "$$ids" ]; then \
-		echo "stopping engine containers…"; \
+		echo "stopping engine containers for $(STACK_LABEL)…"; \
 		docker rm -f $$ids >/dev/null; \
 	fi
 
@@ -87,7 +96,9 @@ stop-engines:
 # cycles.
 prune-broken-engines:
 	@ids=""; \
-	for cid in $$(docker ps -aq --filter label=$(ENGINE_LABEL) 2>/dev/null); do \
+	for cid in $$(docker ps -aq \
+			--filter "label=$(STACK_LABEL)" \
+			--filter "label=$(ENGINE_LABEL)" 2>/dev/null); do \
 		state=$$(docker inspect -f '{{.State.Status}}' $$cid 2>/dev/null); \
 		case "$$state" in \
 			running|restarting) ;; \