Add agent-specific language prompt injection

When running in agent mode (e.g., --agent carmack) in a workspace with
detected languages, inject agent+language-specific prompts from
prompts/langs/<agent>.<lang>.md at the end of the system prompt.

Changes:
- Add AGENT_LANGUAGE_PROMPTS static array for compile-time embedding
- Add get_agent_language_prompt() to look up specific agent+lang combos
- Add get_agent_language_prompts_for_workspace_with_langs() that returns
  both content and matched languages for display
- Update agent_mode.rs to inject prompts and show which languages loaded
- Display format: '✓ carmack: racket language guidance'
- Add tests for new functionality

Uses the same detect_languages() mechanism as regular language prompts
to avoid code-path aliasing.

crates/g3-cli/src/language_prompts.rs

@@ -18,6 +18,13 @@ static LANGUAGE_PROMPTS: &[(&str, &[&str], &str)] = &[
     ),
 ];
 
+/// Embedded agent-specific language prompts.
+/// Format: (agent_name, language_name, prompt_content)
+static AGENT_LANGUAGE_PROMPTS: &[(&str, &str, &str)] = &[
+    // (agent_name, language_name, prompt_content)
+    ("carmack", "racket", include_str!("../../../prompts/langs/carmack.racket.md")),
+];
+
 /// Detect languages present in the workspace by scanning for file extensions.
 /// Returns a list of detected language names.
 pub fn detect_languages(workspace_dir: &Path) -> Vec<&'static str> {
@@ -118,6 +125,46 @@ pub fn list_available_languages() -> Vec<&'static str> {
     LANGUAGE_PROMPTS.iter().map(|(name, _, _)| *name).collect()
 }
 
+/// Get agent-specific language prompt for a specific agent and language.
+pub fn get_agent_language_prompt(agent_name: &str, lang: &str) -> Option<&'static str> {
+    AGENT_LANGUAGE_PROMPTS
+        .iter()
+        .find(|(agent, language, _)| *agent == agent_name && *language == lang)
+        .map(|(_, _, content)| *content)
+}
+
+/// Get agent-specific language prompts for detected languages in the workspace.
+/// Returns formatted content ready for injection into the agent's system prompt.
+#[allow(dead_code)]
+pub fn get_agent_language_prompts_for_workspace(
+    workspace_dir: &Path,
+    agent_name: &str,
+) -> Option<String> {
+    let (content, _) = get_agent_language_prompts_for_workspace_with_langs(workspace_dir, agent_name);
+    content
+}
+
+/// Get agent-specific language prompts for detected languages in the workspace.
+/// Returns both the formatted content and the list of languages that had matching prompts.
+pub fn get_agent_language_prompts_for_workspace_with_langs(
+    workspace_dir: &Path,
+    agent_name: &str,
+) -> (Option<String>, Vec<&'static str>) {
+    let detected = detect_languages(workspace_dir);
+    let mut prompts = Vec::new();
+    let mut matched_langs = Vec::new();
+
+    for lang in detected {
+        if let Some(content) = get_agent_language_prompt(agent_name, lang) {
+            prompts.push(content.to_string());
+            matched_langs.push(lang);
+        }
+    }
+
+    let content = if prompts.is_empty() { None } else { Some(prompts.join("\n\n---\n\n")) };
+    (content, matched_langs)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -166,4 +213,29 @@ mod tests {
         assert!(content.contains("🔧 Language-Specific Guidance"));
         assert!(content.contains("raco"));
     }
+
+    #[test]
+    fn test_carmack_racket_prompt_embedded() {
+        let prompt = get_agent_language_prompt("carmack", "racket");
+        assert!(prompt.is_some());
+        assert!(prompt.unwrap().contains("RACKET-SPECIFIC GUIDANCE"));
+    }
+
+    #[test]
+    fn test_agent_language_prompt_not_found() {
+        let prompt = get_agent_language_prompt("nonexistent", "racket");
+        assert!(prompt.is_none());
+    }
+
+    #[test]
+    fn test_get_agent_prompts_for_workspace() {
+        let temp_dir = TempDir::new().unwrap();
+        let rkt_file = temp_dir.path().join("main.rkt");
+        fs::write(&rkt_file, "#lang racket\n").unwrap();
+
+        let prompts = get_agent_language_prompts_for_workspace(temp_dir.path(), "carmack");
+        assert!(prompts.is_some());
+        let content = prompts.unwrap();
+        assert!(content.contains("RACKET-SPECIFIC GUIDANCE"));
+    }
 }