POC: Define Code Mode file refs

[lt-oai]

Why

Code Mode file handling needs a small shared vocabulary before we can talk about brokered movement, fileParam export, or provider-backed refs.

What

• Adds a FileRef parser in codex-tools.
• Classifies initial provider families: env://, oai_library://, connector://, and other schemes.
• Rejects ambiguous refs without embedding provider-specific secrets or bytes in the ref contract.

Testing

• cargo test -p codex-tools file_ref
• just fmt