Get output from a background shell process.
- Provide shell_id from a background bash execution - Returns current stdout/stderr - Shows if process is still running or completed - Returns cumulative output from process start - Check "done" field to see if process completed - Can call multiple times to see incremental output After starting a server with `run_in_background=true`: ``` shell_id: "abc123" ``` → Returns server output and whether it's still running.