[Feat]: Enforce test_skill.py in test_skill_issuer.py for every registry skill

[rosspeili]

Feature Description

Parent: RFC #156.

Extend tests/test_skill_issuer.py so every directory under skills/ with a manifest.yaml must also contain test_skill.py.

Rollout

• Land after [Feat]: Backfill skill bundle tests for registry skills missing test_skill.py #158 merges (all six missing bundle tests added), or
• Ship with a temporary allowlist of skills still missing bundle tests — remove allowlist in the same PR that closes [Feat]: Backfill skill bundle tests for registry skills missing test_skill.py #158.

Also update

• CONTRIBUTING.md skill checklist — bundle test required
• templates/python_skill/README.md — same rule

Out of scope

• Writing the missing bundle tests ([Feat]: Backfill skill bundle tests for registry skills missing test_skill.py #158)
• CI workflow ([Feat]: CI — add pytest skills/ step alongside existing pytest tests/ #159)
• Issuer/email checks (already in same file — extend, don't rewrite)

Rationale

Docs alone won't stop registry drift. Issuer tests already gate manifest quality; adding bundle-test presence makes the #156 rule machine-enforced.

Implementation Idea

In test_skill_issuer.py, for each skill dir from _discover_skill_dirs(), assert (skill_dir / "test_skill.py").is_file().

Run pytest tests/test_skill_issuer.py locally. No CI workflow change beyond what #159 already does.

[rosspeili]

This looks ready to implement on current main: #158 backfill is merged and all 10 registry skills have test_skill.py.

No allowlist needed anymore — assert every manifest.yaml directory also has test_skill.py in tests/test_skill_issuer.py.