Scenarios install their Galaxy dependencies with `force: true`, so two roles
running at once re-extract the same collections and roles into ~/.ansible and
pull them out from under each other mid-play. It surfaces as a collection that
was working moments earlier going missing:

  the connection plugin 'community.docker.docker' was not found

Found while running five scenarios in parallel, where it cost a run.

ANSIBLE_HOME relocates both `collections/` and `roles/`, so one variable covers
both halves; the scenarios' ANSIBLE_ROLES_PATH workaround now follows it rather
than hardcoding ~/.ansible/roles. Left alone if already set, and unset in CI,
where each role runs in its own job and has nothing to collide with.

Verified by removing var/molecule-ansible-home entirely and running
matrix-alertmanager-receiver from cold: green through idempotence, with the
collections and roles landing under the per-role directory - which also shows
nothing was quietly relying on the shared ~/.ansible being populated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT

Three things that would not have scaled to 70 roles:

- The Python and Ansible dependency pins were about to be copied into every
  role. They now live once in molecule-shared/, which scenarios reference
  relatively, so they cannot drift apart.
- The helper container images used for probing were hardcoded inline. They
  are pinned once in molecule-shared/vars.yml, carry `# renovate:`
  annotations, and a custom manager in .github/renovate.json keeps them
  current - verified with a local Renovate dry run, which offers
  curl 8.11.1 -> 8.21.0 and python 3.13 -> 3.14-alpine. Seventy invisible
  hardcodes is the blindness class we have been removing elsewhere.
- Running a scenario meant knowing the venv and cd incantation. `just
  molecule <role>` does it, and with no argument lists the roles that have
  a scenario.

Molecule is deliberately not wired into prek: a run takes minutes, pulls
images and needs Docker, which is fine on request and not fine per commit.

docs/molecule-testing.md covers how to run and write these, including the
four things a role here needs that a standalone role does not. AGENTS.md
points at it rather than carrying the detail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>