[codi-rs][P0] Reconcile readiness/security/changelog documentation with actual repo state

[laynepenney]

Parent tracker: #294

Problem

Current docs contain inconsistencies (readiness stats, Rust edition, security contact, changelog links), reducing trust in release readiness claims.

Scope

• Update docs/PRODUCTION_READINESS_PLAN.md claims to match current state
• Align security contact across README.md and SECURITY.md
• Fix changelog compare/release links to correct repo owner
• Remove stale branch/status claims

Acceptance Criteria

• Docs are internally consistent and accurate
• Links resolve correctly to the live laynepenney/codi-rs repo
• No stale "all green" claims unless verified