Remove machine mode entirely from g3

- Delete machine_ui_writer.rs
- Remove --machine CLI flag from cli_args.rs
- Remove run_machine_mode(), run_interactive_machine(), run_autonomous_machine() functions
- Remove handle_machine_command() function
- Simplify OutputMode enum to just use SimpleOutput directly
- Simplify SimpleOutput struct (remove machine_mode field)
- Remove machine_mode parameter from setup_workspace_directory()
- Remove test_machine_option_accepted test
- Disable ACD by default in agent_mode (requires --acd flag)
- Change 'memory checkpoint' message formatting
- Remove dehydration status message

crates/g3-cli/src/agent_mode.rs

@@ -209,9 +209,6 @@ pub async fn run_agent_mode(
     // This prompts the LLM to save discoveries to project memory after each turn
     agent.set_auto_memory(true);
 
-    // Enable ACD in agent mode for longer sessions
-    agent.set_acd_enabled(true);
-
     // If resuming a session, restore context and TODO
     let initial_task = if let Some(ref incomplete_session) = resuming_session {
         // Restore the session context