docs(readme): document persistent user configuration

[JMHSV]

Summary

• document ~/.cli-proxy-api/config.yaml as the persistent user-owned override file
• explain that VibeProxy generates merged-config.yaml and that users should not edit it directly
• add a tested Codex text.verbosity: low payload-default example
• clarify default versus override behavior and automatic reloads

Verification

• git diff --check
• README fence and example assertions
• manually verified with VibeProxy 1.8.243 that editing config.yaml regenerated merged-config.yaml without restarting and that a request omitting text.verbosity returned verbosity: low