Enforce the datamachine_ prefix in CI

[chubes4]

Summary

• add a token-based production PHP prefix audit
• cover symbols, variables, hooks, option and transient keys, meta keys, constants, taxonomy slugs, and database tables
• avoid documentation, fixture, vendor, and unrelated substring noise
• run the repository policy directly in CI with focused positive and negative fixtures

How to test

1. Run php tests/prefix-policy-audit.php.
2. Confirm the legacy fixture reports every protected identifier surface.
3. Confirm valid datamachine_ identifiers and prose produce no findings.

Closes #2933