Document compatibility surfaces kept outside release flow

[shiny-code-bot]

Summary

Make the compatibility we keep explicit, so future cleanup does not confuse external tooling support with Every Code release-channel support.

Finish Line

Remaining npm/Homebrew/CODEX/codex-rs references are clearly scoped to external tools, upstream compatibility, or temporary local migration, not Every Code release distribution.

Current Status

State: Waiting
Current evidence: cleanup-direct-binary-updates leaves npm/Homebrew mentions only where they are non-release compatibility or tooling: external agent installs, validation tool install hints, CODEX compatibility, codex-rs/upstream mirror policy, and a temporary old Homebrew symlink cleanup script.
Next action: After #298 deletion PR lands, add or update the minimal docs/policy note that remaining npm/Homebrew/CODEX/codex-rs references are not Every Code release channels.
Not started because the exact remaining surface depends on the codex-cli/ deletion follow-up.

Acceptance Criteria

• External agent and validation-tool npm/Homebrew references are preserved and identified as non-release tooling.
• CODEX_* compatibility names remain documented where required.
• codex-rs remains documented as read-only upstream mirror.
• Temporary Homebrew symlink cleanup is either retained with legacy-migration wording or removed after a decision.
• Release/update docs do not list npm/Homebrew as supported or deferred channels.
• ./build-fast.sh passes if docs or code change.

Relationships

Parent: #294.
Supports the whole cleanup workstream by preventing accidental removal of needed non-release compatibility.

• related: Plan Codex-base substrate migration for Every Code #384 - Plan Codex-base substrate migration for Every Code #384