docs: add Standby branding and complete presentation session evidence

[garykocsis]

Summary

• add approved Standby branding assets under assets/brand/
• add the Standby mark above the root README title
• preserve existing README prose, diagrams, verification evidence, and protocol framing
• add Session 19 branding integration prompt and implementation log
• complete the Session 18 ChatGPT retrospective record

Scope

This is documentation and presentation-layer work only.

No protocol code, tests, frontend implementation, project status, or canonical specification artifacts were changed.

Verification

• existing README prose unchanged
• frozen Mermaid diagrams unchanged
• verification evidence unchanged
• production code unchanged
• frontend unchanged
• docs/project-status.md unchanged
• approved branding assets copied without redesign
• no test rerun required for these non-executable changes