[codex] Clarify Qwen mmproj naming in docs

[amariichi]

Summary

• update the README to avoid documenting a local-only renamed Qwen mmproj filename as if it were the canonical one
• use models/mmproj-F32.gguf in the example command
• add a note that Qwen requires its own mmproj and that users may rename it locally if filenames would conflict

Validation

• documentation-only change