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/display.rs

@@ -64,6 +64,7 @@ impl LoadedContent {
     }
 
     /// Create with explicit include prompt filename.
+    #[allow(dead_code)] // Used in tests, may be useful for future callers
     pub fn with_include_prompt_filename(mut self, filename: Option<String>) -> Self {
         if self.include_prompt_filename.is_some() {
             self.include_prompt_filename = filename;