chore: prepare repo for Automattic ownership

[chubes4]

Summary

• Updates Composer metadata, plugin header ownership, and README wording for the Automattic-owned repository.
• Adds a minimal GitHub Actions workflow that runs the existing Composer smoke-test coverage on PRs and main pushes.

Tests

• composer test
• composer validate --strict

Closes #2

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting and testing repository metadata/CI readiness changes under Chris's review.