Rust-specific readability guidance for the carmack agent including:
- let...else example for shallow control flow
- Async: don't block the runtime (tokio::fs, spawn_blocking, Send)
- Visibility: prefer pub(crate), private fields with accessors
- Generics: impl Trait over explicit params, avoid complex where clauses
- Improved iterator guidance: if you need a comment, use a loop
- UTF-8 string slicing warnings
- Ownership/lifetime pragmatism
- Anti-patterns: no macros/typestate/proc-macros unless already in repo
Also adds Rust detection to LANGUAGE_PROMPTS (empty base prompt,
agent-specific prompts handle the guidance).
