Define Rust authority and Python retirement plan

[tcballard]

Summary

• accept ADR-118: Rust becomes the sole RAC engine authority
• inventory every runtime, API, install, Homebrew, and rac-ci migration surface
• add the R0-R6 retirement roadmap and clarify the historical native cutover plan
• regenerate managed AGENTS.md guidance

Validation

• rac validate rac/ --json: 427 valid, 0 invalid, OKF conformant
• rac relationships rac/ --validate --json: 2,579 checked, 0 issues
• rac review rac/ --json: ok, health 95, no priority 1 or 2 findings
• rac gate rac/ --json: ok, no blocking findings
• rac export rac/ --agent-rules --check --client agents: in sync
• git diff --cached --check

Scope

This is the R0 authority and inventory boundary only. Runtime migration work remains split across R1-R6 so contract extraction, Python-only surfaces, CI authority, distribution, deprecation, and deletion can be reviewed independently.