fix(codex): keep unreadable shim checks advisory

[luvs01]

Summary

• Keep the post-install Codex shim readiness check advisory when the Codex config cannot be read or changes between existence and read checks.
• Reuse the existing getCodexRoutingKind() fallback so the install succeeds and emits the generic routing-verification warning instead of failing after the shim has already been written.
• Add a cross-platform end-to-end regression with config.toml represented by an unreadable directory and platform-appropriate Codex launchers.

Verification

• bun test tests/codex-shim-readiness.test.ts — 6 passed, 0 failed.
• bun run typecheck
• bun run privacy:scan
• git diff --check
• Independent focused diff review: no P0-P3 findings.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed. (No documentation change is needed for this failure-path fix.)
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

Review readiness checklist

• All CI tests are green on my local testing.

• I pushed my PR to the latest dev commit.

• I resolved all correct Codex and CodeRabbit findings.

• My PR is ready for review.

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between d517161 and 1eb8ce3.

📒 Files selected for processing (2)

• src/cli/codex-shim-readiness.ts
• tests/codex-shim-readiness.test.ts

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

✅ READY

• all PR quality gates passed; the review readiness checklist is complete.

Review readiness checklist

• ✅ All CI tests are green on my local testing.
• ✅ I pushed my PR to the latest dev commit.
• ✅ I resolved all correct Codex and CodeRabbit findings.
• ✅ My PR is ready for review.

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Maintainers: @lidge-jun @Ingwannu @Wibias

[Wibias]

Please rebase this PR onto the latest dev (e8ce2b93dd3ef8074d96dee407704c5dc06e2699) before merge, then rerun the exact-head CI checks.

[Wibias]

Thanks @luvs01 — good failure-path hardening. Keeping the post-install readiness check advisory avoids turning an unreadable/racy Codex config into a false install failure, while preserving the existing warning behavior. 🙏