Define CODE and CODEX compatibility policy

[cbusillo]

Summary

• document CODE/CODEX environment and path compatibility policy in the upstream import policy
• link the policy from config docs and repository guidance
• clarify preserved CODEX_API_KEY and CODEX_TUI_* debug variable posture without changing runtime behavior

Validation

• rg -n "CODE And CODEX Compatibility Policy|CODEX_TUI|CODEX_API_KEY|CODE_HOME|CODEX_HOME|scoped migration|alias|compatibility" AGENTS.md docs/upstream-import-policy.md docs/config.md docs/exec.md docs/auto-drive.md
• git diff --check
• ./build-fast.sh

Refs #179
Refs #183