Add language-specific prompt injection for toolchain guidance

- Add language_prompts module that auto-detects programming languages in workspace
- Scan for language files with depth limit (2) to inject relevant toolchain prompts
- Add prompts/langs/ directory for language-specific markdown files
- Include Racket/raco toolchain guidance as first language prompt
- Update combine_project_content() to accept language_content parameter
- Integrate language detection into main CLI flow and agent mode
- Update project memory with new feature documentation

crates/g3-cli/src/agent_mode.rs

@@ -146,6 +146,7 @@ pub async fn run_agent_mode(
         agents_content_opt,
         readme_content_opt,
         memory_content_opt,
+        crate::language_prompts::get_language_prompts_for_workspace(&workspace_dir),
         &workspace_dir,
     );