refactor: Remove hardcoded Web Research section from system prompt

- Web Research instructions now come from skills/research/SKILL.md
- Skills are dynamically loaded and injected via generate_skills_prompt()
- Remove test_both_prompts_have_web_research test (no longer applicable)
- Remove unused G3Status::research_complete() function

This completes the externalization of research as a skill.

crates/g3-core/src/prompts.rs

@@ -258,15 +258,6 @@ mod tests {
         assert!(non_native.contains("# Workspace Memory"));
     }
 
-    #[test]
-    fn test_both_prompts_have_web_research() {
-        let native = get_system_prompt_for_native();
-        let non_native = get_system_prompt_for_non_native();
-        
-        assert!(native.contains("# Web Research"));
-        assert!(non_native.contains("# Web Research"));
-    }
-    
     #[test]
     fn test_native_prompt_loaded_from_file() {
         // Verify the include_str! macro successfully loads the file