[codex] Add Project Instructions governance addendums

[ToolboxAid]

Summary

• Added a canonical repository structure governance addendum at project-instructions/addendums/canonical-repository-structure.md.
• Added a platform development standards addendum at project-instructions/addendums/platform-development-standards.md.
• Appended both addenda to the Project Instructions README index so they are discoverable.

Impact

This documents required folder structure for new and modified code, plus repository-wide standards for PostgreSQL usage, referenced entity protection, table-first creator tool design, shared component governance, and standards enforcement.

Validation

• git diff --cached --check passed before each commit.
• Docs-only changes; no runtime tests were run.

[ToolboxAid]

OWNER-approved cleanup: closing as superseded. Current Project Instructions already carry this governance through the split addendum set: canonical repository structure, test structure standardization, legacy migration policy, PostgreSQL-only persistence, referenced asset protection, and table-first UI. A non-destructive merge probe also showed real protected-governance conflicts in README/canonical-structure content, so this PR should not be partially applied. Branch retained; no branch deletion performed.