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/lib.rs

@@ -12,6 +12,7 @@ mod agent_mode;
 mod autonomous;
 mod cli_args;
 mod coach_feedback;
+mod commands;
 mod display;
 mod interactive;
 mod simple_output;