feat: enhance ecotone-contributor skill with test-first rules and broader trigger

[dgafka]

Why is this change proposed?

The contributor skill previously only triggered for environment setup and PR workflows. It needed to guide contributors on every code change, and enforce test-first development using Ecotone Lite with a userland perspective — ensuring all new tests serve as real-world usage examples for framework users.

Description of Changes

• Broadened the skill trigger to activate on any code change (features, bug fixes, refactors)
• Added mandatory test-first rule (Section 0) requiring Ecotone Lite tests before production code
• Tests must follow the userland perspective — each test reads like an end-user usage example
• Tests must use inline anonymous classes defined within the test method, no separate fixtures
• No static properties or methods allowed in test classes — instance only
• Includes a complete example skeleton demonstrating all conventions

Pull Request Contribution Terms

• I have read and agree to the contribution terms outlined in CONTRIBUTING.