refactor(g3-cli): extract commands module and fix test organization

- Extract handle_command() from interactive.rs to new commands.rs module
  (320 lines, 15 match arms for /help, /compact, /thinnify, etc.)
- Fix orphaned tests in completion.rs that were outside mod tests block
- Add #[allow(dead_code)] to with_include_prompt_filename() (used in tests)
- interactive.rs reduced from 595 to 290 lines

Agent: fowler

crates/g3-cli/src/completion.rs

@@ -509,7 +509,6 @@ mod tests {
         println!("Line: '{}', start: {}, completions: {}", line, start, completions.len());
         assert_eq!(completions.len(), 0, "Quoted non-path should not trigger completion");
     }
-}
 
     #[test]
     fn test_resume_completion_lists_sessions() {
@@ -561,3 +560,4 @@ mod tests {
         // The important thing is it doesn't panic
         println!("list_sessions returned {} sessions", sessions.len());
     }
+}