Add a start flag, and let --check survive a first deploy

-e start=false (mode sync in CI) lands the payload and starts nothing, for
migrations. A check run on a never-deployed stack now skips Compose with a note
instead of failing on the missing directory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ansible/roles/compose_stack/defaults/main.yml

@@ -51,3 +51,10 @@ stack_pull: "{{ pull | default('policy') }}"
 # Compose build policy. `policy` builds only when the image is missing; stacks
 # that build from a Dockerfile in their payload want `always` on an update run.
 stack_build: policy
+
+# Whether to start the stack at all. False syncs the payload, creates the
+# directories and the networks, and stops before Compose — the shape a migration
+# needs, where data has to be moved into a bind mount after the files land and
+# before anything opens them. Defaults to the repo-wide `start` flag, so
+# `-e start=false` works on every playbook without editing any of them.
+stack_start: "{{ start | default(true) | bool }}"