fix: drop RCT_NEW_ARCH_ENABLED flag injection for pod install

[floydkim]

Summary

This PR removes CLI-side injection of RCT_NEW_ARCH_ENABLED env var during iOS pod install.

Background:

• The previous behavior forced RCT_NEW_ARCH_ENABLED (0 or 1) from CLI when running pod installation paths such as --only-pods.
• Based on maintainer feedback and current RN defaults, the CLI should not force this flag and should let the project/RN configuration drive architecture behavior.

What changed:

• Removed RCT_NEW_ARCH_ENABLED from installPods pod install environment.
  • Removed now-unused newArchEnabled plumbing in the iOS pod resolution flow (run/build -> resolvePods -> installPods).

And also updated CONTRIBUTING.md to include cli-config-android and cli-config-apple in the yarn link example, so local-link tests also cover config package changes.

Test Plan

Linked local CLI validation (per CONTRIBUTING.md)

In these cases, --only-pods followed project architecture behavior as expected.

• RN 0.81.5 + Reanimated v4
• RN 0.81.5 project without Reanimated (Podfile configured with RCT_NEW_ARCH_ENABLED=0)
• RN 0.84.0 template project

Checklist

• Documentation is up to date.
• Follows commit message convention described in CONTRIBUTING.md.
• For functional changes, my test plan has linked these CLI changes into a local react-native checkout (instructions).