[codex] Add audit evidence notes

[roadynet]

Summary

• Add an evidence index under docs/evidence/.
• Document PHPStan baseline reduction evidence from 56 to 17 entries.
• Add private codebase verification notes, including targeted PHPStan checks, syntax checks, and the local Windows/P-drive limitation.
• Link the evidence notes from README, the PHPStan audit page, and the quality report.

Validation

• Ran git diff --cached --check before committing.
• Verified links with repository search output.

Notes

This adds portfolio-safe evidence only. It does not publish private application source code, secrets, production config, or user data.