fix(compose): keep galaxy.stack label on containers only

The previous commit stamped `galaxy.stack=<value>` on services,
volumes, and networks. Putting it on volumes/networks changes their
compose config-hash on every label revision, so `docker compose up`
tries to recreate them — which on the long-lived dev environment
either destroys the postgres data volume or deadlocks while trying
to remove `galaxy-dev-internal` with containers still bound to it.
Observed live: run #184 hung in compose recreate after the three
stateful services were stopped, with no recovery.

Containers alone are sufficient for the cleanup contract (we filter
containers, not volumes or networks). Roll back the label on volumes
and networks in both compose files and capture the rule in
docs/ARCHITECTURE.md so the next contributor does not reintroduce it.

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

tools/local-dev/docker-compose.yml

@@ -218,11 +218,10 @@ services:
 networks:
   galaxy-net:
     name: galaxy-local-dev-net
-    labels:
-      galaxy.stack: local-dev
 
+# See note in tools/dev-deploy/docker-compose.yml — labels live only
+# on services (containers), not on volumes or networks, to keep the
+# compose config-hash for stateful resources stable across deploys.
 volumes:
   postgres-data:
     name: galaxy-local-dev-postgres-data
-    labels:
-      galaxy.stack: local-dev