fix(ci): pre-start E2E compose stack before pytest (ADCP_TESTING=true)

[mkostromin-sigma]

Summary

Implement the CI half of #1666:

1. E2E job builds creative-agent + compose images, then `compose up -d --wait` (healthchecks gate postgres, creative-agent, adcp via proxy).
2. Run pytest with `ADCP_TESTING=true` so `docker_services_e2e` is verify-only.
3. Keep `--timeout=300` for test bodies; session setup no longer does cold build under that cap.
4. Free disk before build (same class as fix(ci): BDD In-Network ENOSPC during tox uv sync after #1613/#1634 #1662) if needed for image headroom.
5. Arch guard: E2E job must set `ADCP_TESTING=true` and must not rely on fixture-owned cold build.

Closes this leaf; parent #1666.

Acceptance

• Cold CI E2E: no setup killed by `Timeout (>300.0s)`
• All required services healthy before pytest starts
• Guard pins `ADCP_TESTING=true` + pre-start steps on `e2e-tests`

[mkostromin-sigma]

PR: #1669