docs: clarify hybrid login relay mode

[aDragon0707]

Summary

• Clarify that relay injection is a hybrid mode: official ChatGPT/Codex login keeps account features and plugin access, while the relay profile only controls model request routing.
• Document provider-neutral preflight checks before applying relay injection.
• Emphasize secret-safe diagnostics and config backup before switching modes.

Notes

This is intentionally provider-neutral and does not document any specific relay vendor. It also does not propose bypassing official login; the official login state remains the source of Codex App account/plugin capabilities.

Testing

• git diff --check