Surface the RightMemory semantic model in README

[RightL]

Summary

• add a concise semantic-model section near the top of the README
• distinguish Memory, Pursuit, and Agent Corrections, including what should not be stored
• expose direct links to the existing canonical rules and schema
• rename the later model section to focus on document representation and remove repeated semantic definitions

Scope

README-only documentation change. No runtime files, rule documents, or code paths are changed.

Validation

• final branch diff contains only README.md
• verified the new top-level section and the revised document-model introduction
• git diff --check passed in the edit workflow