Fix empty Language-Specific Guidance header in system prompt

When a Rust-only workspace was detected, the Language-Specific Guidance
header was appearing with no content because Rust has an empty prompt
string (agent-specific prompts handle Rust instead).

The fix filters out empty prompt strings in get_language_prompts_for_workspace()
so the header only appears when there's actual guidance content.

Added test to verify Rust-only workspaces return None.

crates/g3-cli/src/language_prompts.rs

@@ -112,7 +112,9 @@ pub fn get_language_prompts_for_workspace(workspace_dir: &Path) -> Option<String
     let mut prompts = Vec::new();
     for lang in detected {
         if let Some(content) = get_language_prompt(lang) {
-            prompts.push(content);
+            if !content.is_empty() {
+                prompts.push(content);
+            }
         }
     }
 
@@ -244,4 +246,15 @@ mod tests {
         let content = prompts.unwrap();
         assert!(content.contains("obvious, readable Racket"));
     }
+
+    #[test]
+    fn test_rust_only_returns_none() {
+        // Rust has an empty prompt, so a Rust-only workspace should return None
+        let temp_dir = TempDir::new().unwrap();
+        let rs_file = temp_dir.path().join("main.rs");
+        fs::write(&rs_file, "fn main() {}").unwrap();
+
+        let prompts = get_language_prompts_for_workspace(temp_dir.path());
+        assert!(prompts.is_none(), "Rust-only workspace should return None since Rust has no base prompt");
+    }
 }

prompts/system/.#native.md

@@ -0,0 +1 @@
+dhanji@Dhanjis-Mac-Studio.local.20760:1769746925