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/README.md

@@ -135,17 +135,20 @@ exec galaxy-mailpit wget -qO- localhost:8025/messages` and similar.
 ## Persistent state and schema changes
 
 The dev Postgres volume `galaxy-dev-postgres-data` survives redeploys.
-Until the pre-production migration rule is lifted, every
-backward-incompatible change to `backend/internal/postgres/migrations/00001_init.sql`
-needs a manual wipe before the next deploy succeeds:
+Schema deltas land as additive, sequence-numbered migration files
+(`backend/internal/postgres/migrations/0000N_*.sql`) and `pressly/goose`
+applies them on backend startup without operator action.
+
+Use `make -C tools/dev-deploy clean-data` only when you deliberately
+want a fresh database (debugging schema drift, exercising the
+bootstrap path from scratch, etc.):
 
 ```sh
 make -C tools/dev-deploy clean-data
 make -C tools/dev-deploy up
 ```
 
-This is the same caveat as `tools/local-dev/`, just with a different
-volume name.
+The same volume-persistence model applies to `tools/local-dev/`.
 
 ## Make targets
 
@@ -183,13 +186,30 @@ See [`KNOWN-ISSUES.md`](KNOWN-ISSUES.md) for symptoms that surface
 in the long-lived dev environment but are not yet fixed (currently:
 the sandbox game flipping to `cancelled` after a redispatch).
 
+## Deployment cadence
+
+This environment is single-tenant: one live deployment, redeployed by
+the `dev-deploy.yaml` workflow on every merge into `development`. PR
+branches do not auto-deploy here — pushes to `feature/*` only run the
+test workflows (`go-unit`, `ui-test`, `integration`).
+
+To put a feature branch on the shared dev environment before its PR
+merges (e.g. to validate a UI flow against the real Caddy edge), run
+the workflow manually:
+
+1. Push the branch (`git push gitea HEAD`).
+2. Gitea UI → **Actions → Deploy · Dev → Run workflow**, pick the
+   feature ref.
+
+The deploy is idempotent — when the PR later merges into
+`development`, the regular push trigger fires the same packaging and
+healthcheck steps, overwriting whatever the manual dispatch left
+behind. There is no separate state to clean up between the two paths.
+
 ## Relationship to other infrastructure
 
 - `tools/local-dev/` — single-developer playground, host-port mapped,
   Vite dev server on the side. Recommended for active UI work.
-- `tools/local-ci/` — Gitea + act runner for **fallback** workflow
-  testing without `gitea.lan`. Optional, not part of the per-stage CI
-  gate anymore.
 - `.gitea/workflows/dev-deploy.yaml` — the CI side of this stack:
   builds images, seeds the UI volume, runs `docker compose up -d` on
   every merge into `development`. The Makefile in this directory is