docs: align doc references with compose.yml rename (code-reviewer M1)

Three docs still referenced the old docker-compose.yml path. Replace
with compose.yml so a future reader cloning at this hash finds the
file at the documented path.

- CHANGELOG.md:31 — backend skeleton recap line.
- docs/architecture.md:28 — deployment artifacts note (D-010 scope).
- tasks/todo.md:9 — B0.1 task description.

Also adds a "CI follow-ups (sprint 1+)" section to tasks/todo.md
capturing the 3 MINOR + 6 NIT deferred from code-reviewer's review
of chore/podman-and-ci, plus a FERNET-KEY tracker for the secret
provisioning before c2_credential.config_fernet (D-004) is wired.

docs/architecture.md

@@ -25,7 +25,7 @@ mimic/
 
 Deployment artifacts (Ansible playbook, prod compose) live outside the repo
 in the RT infra repo (D-010). Mimic ships only Dockerfiles and a dev
-`docker-compose.yml`.
+`compose.yml`.
 
 ## Backend module tree