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/dev-deploy/Makefile

@@ -4,6 +4,7 @@
 
 REPO_ROOT := $(realpath $(CURDIR)/../..)
 ENGINE_IMAGE := galaxy-engine:dev
+STACK_LABEL := galaxy.stack=dev-deploy
 ENGINE_LABEL := org.opencontainers.image.title=galaxy-game-engine
 # Game-state root lives under the invoking user's home by default so
 # `make up` works without sudo. Override `GALAXY_DEV_GAME_STATE_DIR`
@@ -93,12 +94,14 @@ psql:
 
 clean-data:
 	@echo "Stopping containers and engines, then wiping volumes + game-state…"
-	@ids=$$(docker ps -aq --filter label=$(ENGINE_LABEL)); \
+	$(COMPOSE) down -v
+	@ids=$$(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
-	$(COMPOSE) down -v
 	@if [ -d "$(GALAXY_DEV_GAME_STATE_DIR)" ]; then \
 		echo "wiping $(GALAXY_DEV_GAME_STATE_DIR)…"; \
 		docker run --rm -v "$(GALAXY_DEV_GAME_STATE_DIR):/state" alpine sh -c 'rm -rf /state/*' 2>/dev/null \