chore: Automattic repo readiness and CI

[chubes4]

Goal

Make the moved Automattic/agents-api repository look and behave like an Automattic-owned standalone WordPress substrate repo, rather than a freshly copied extraction artifact.

Current state

• Repository has moved to https://github.com/Automattic/agents-api.
• Local checkout may still point at https://github.com/Extra-Chill/agents-api.git.
• composer.json still uses extra-chill/agents-api.
• Plugin header still says Author: Extra Chill.
• There is no .github/workflows/* CI workflow.
• There is no homeboy.json.
• The smoke tests already pass with composer test.

Desired outcome

• Composer/package metadata is ready for Automattic ownership.
• Plugin header and README ownership wording no longer imply Extra Chill ownership.
• A minimal CI workflow runs the existing smoke tests on PRs.
• Optional Homeboy config is added if it matches the repo conventions.
• No runtime behavior changes.

Notes

• Do not bump versions manually.
• Do not add Data Machine as a dependency.
• Keep this PR focused on repo ceremony and ownership/readiness, not source-tree refactors.

Acceptance criteria

• composer test passes.
• CI exists and runs equivalent smoke coverage.
• Repo metadata no longer points at Extra Chill as the owner.
• PR body includes AI assistance disclosure.