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/docker-compose.yml

@@ -19,11 +19,15 @@
 # can log in without touching Mailpit. Real codes still arrive in
 # Mailpit; both paths coexist.
 
+name: galaxy-local-dev
+
 services:
   postgres:
     image: postgres:16-alpine
     container_name: galaxy-local-dev-postgres
     restart: unless-stopped
+    labels:
+      galaxy.stack: local-dev
     environment:
       POSTGRES_USER: galaxy
       POSTGRES_PASSWORD: galaxy
@@ -45,6 +49,8 @@ services:
     image: redis:7-alpine
     container_name: galaxy-local-dev-redis
     restart: unless-stopped
+    labels:
+      galaxy.stack: local-dev
     command:
       - redis-server
       - --requirepass
@@ -68,6 +74,8 @@ services:
     image: axllent/mailpit:v1.21
     container_name: galaxy-local-dev-mailpit
     restart: unless-stopped
+    labels:
+      galaxy.stack: local-dev
     ports:
       - "${LOCAL_DEV_MAILPIT_PORT:-8025}:8025"
     networks:
@@ -86,6 +94,8 @@ services:
     image: galaxy/backend:local-dev
     container_name: galaxy-local-dev-backend
     restart: unless-stopped
+    labels:
+      galaxy.stack: local-dev
     user: "0:0"
     depends_on:
       postgres:
@@ -102,6 +112,7 @@ services:
       BACKEND_SMTP_FROM: "galaxy-backend@galaxy.local"
       BACKEND_SMTP_TLS_MODE: none
       BACKEND_DOCKER_NETWORK: galaxy-local-dev-net
+      BACKEND_STACK_LABEL: local-dev
       BACKEND_GAME_STATE_ROOT: /tmp/galaxy-game-state
       BACKEND_GEOIP_DB_PATH: /var/lib/galaxy/geoip.mmdb
       BACKEND_NOTIFICATION_ADMIN_EMAIL: admin@galaxy.local
@@ -144,6 +155,8 @@ services:
     image: galaxy/gateway:local-dev
     container_name: galaxy-local-dev-gateway
     restart: unless-stopped
+    labels:
+      galaxy.stack: local-dev
     depends_on:
       backend:
         condition: service_healthy
@@ -205,7 +218,11 @@ services:
 networks:
   galaxy-net:
     name: galaxy-local-dev-net
+    labels:
+      galaxy.stack: local-dev
 
 volumes:
   postgres-data:
     name: galaxy-local-dev-postgres-data
+    labels:
+      galaxy.stack: local-dev