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.

CHANGELOG.md

@@ -28,7 +28,7 @@ UX wireframes (mock data). No real connector, no reporting until PR1/PR2/PR3 lan
 #### Backend skeleton (`feature/backend-skeleton`)
 
 - `backend/` Python 3.12+ project: `pyproject.toml` (ruff, mypy strict, pytest, coverage 70 %),
-  `Makefile` (Docker/Podman auto-detect), multi-stage `Dockerfile`, `docker-compose.yml` for
+  `Makefile` (Docker/Podman auto-detect), multi-stage `Dockerfile`, `compose.yml` for
   Postgres dev DB, `.env.example`.
 - Full §8 data model in SQLAlchemy 2 typed mapped classes: `engagement`, `c2_credential`,
   `host`, `user`, `group`, `permission`, `group_permission`, `user_group`,