Add carmack.rust.md agent-specific language prompt

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).

crates/g3-cli/src/language_prompts.rs

@@ -11,6 +11,11 @@ use std::path::Path;
 /// The key should match common file extensions or language identifiers.
 static LANGUAGE_PROMPTS: &[(&str, &[&str], &str)] = &[
     // (language_name, file_extensions, prompt_content)
+    (
+        "rust",
+        &[".rs"],
+        "",  // No base Rust prompt; agent-specific prompts handle this
+    ),
     (
         "racket",
         &[".rkt", ".rktl", ".rktd", ".scrbl"],
@@ -23,6 +28,7 @@ static LANGUAGE_PROMPTS: &[(&str, &[&str], &str)] = &[
 static AGENT_LANGUAGE_PROMPTS: &[(&str, &str, &str)] = &[
     // (agent_name, language_name, prompt_content)
     ("carmack", "racket", include_str!("../../../prompts/langs/carmack.racket.md")),
+    ("carmack", "rust", include_str!("../../../prompts/langs/carmack.rust.md")),
 ];
 
 /// Detect languages present in the workspace by scanning for file extensions.