Feat/issue 160 enforce bundle tests

[rosspeili]

Description

Adds a framework test so every skill under skills/ with a manifest.yaml must also ship test_skill.py. This turns the #156 bundle-test rule from docs-only into a CI gate via the existing tests/test_skill_issuer.py harness.

Also documents the enforcement in CONTRIBUTING, TESTING.md, and the skill template.

Fixes #160

Type of change

• Framework Feature / RFC Updates — registry test enforcement ([RFC]: Testing architecture - bundle, framework, and maintainer tests in CI #156)

Checklist

• Code follows the Agent Code of Conduct
• pytest tests/test_skill_issuer.py run locally (4 passed); black / flake8 clean on changed files
• CHANGELOG.md updated under [Unreleased]
• examples/README.md — N/A

Related

• [Feat]: Enforce test_skill.py in test_skill_issuer.py for every registry skill #160 — enforce bundle test presence
• [RFC]: Testing architecture - bundle, framework, and maintainer tests in CI #156 — testing architecture RFC (parent)
• [Feat]: Backfill skill bundle tests for registry skills missing test_skill.py #158 — bundle test backfill (prerequisite, merged)