Share the Molecule scaffolding and add just molecule

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>

molecule-shared/requirements.txt

@@ -0,0 +1,4 @@
+ansible==14.1.0
+molecule==26.6.0
+molecule-plugins==26.7.8
+docker==7.2.0