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/cli_args.rs

@@ -55,10 +55,6 @@ pub struct Cli {
     #[arg(long)]
     pub chat: bool,
 
-    /// Enable machine-friendly output mode with JSON markers and stats
-    #[arg(long)]
-    pub machine: bool,
-
     /// Override the configured provider (anthropic, databricks, embedded, openai)
     #[arg(long, value_name = "PROVIDER")]
     pub provider: Option<String>,