ci(security): rename to nightly-security.yml and run daily

[sbaerlocher]

Summary

Renames weekly-security.yml → nightly-security.yml ("Nightly Security Scan") and switches to a daily cron 0 2 * * *.

Why

Per the arillso/sbaerlocher repo-standard visibility rule: public repos get free GitHub Actions minutes → security scan runs daily; private repos run it weekly to save paid minutes. ansible.container is public → daily. (Standard note updated 2026-06-27.)

Test plan

• CI green

[claude]

Thanks — the rename + daily cron switch and the AGENTS.md updates are consistent and correct (0 2 * * * = daily 02:00 UTC, name "Nightly" matches the cadence).

One blocking issue: the ## [Unreleased] section of CHANGELOG.md (lines 82–84) still documents the reverse of this change — it records renaming nightly-security.yml → weekly-security.yml with a weekly cron. Since that entry is in the same unreleased cycle this PR reverses, the unreleased changelog now describes a file/cadence that no longer exists. Please update it so the notes match reality (per AGENTS.md, CHANGELOG.md must stay current).

Nit (pre-existing, non-blocking): in the AGENTS.md file tree the # Scheduled daily secret scan comment isn't column-aligned with the neighbouring entries — same single-space gap the old weekly-security.yml line had, so not introduced here.

[claude]

All raised issues have been addressed. The CHANGELOG entry now accurately reflects the nightly security scan cadence, and the docker_compose_v2 version bump (5.1.4 → 5.2.0) is consistent across defaults and argument_specs.