fix(run): cross-platform browser opening and secure token input

[waklo]

Summary

Improve the run_rl_swarm.sh UX and startup reliability across platforms.

Motivation

• The script currently uses open to launch http://localhost:3000, which only exists on macOS. On Linux this fails and the user has to open the URL manually.
• The Hugging Face access token prompt echoes the token back to the terminal, which is not ideal from a UX and security perspective.

Changes

• Detect xdg-open (Linux) and open (macOS) and use whichever is available to launch http://localhost:3000. If neither is available, fall back to the existing manual message.
• Switch the Hugging Face token prompt to read -s so the token is not echoed to the terminal, adding a newline afterwards for a clean prompt.

Testing

• Manually ran run_rl_swarm.sh on:
  • macOS: confirmed the browser opens via open when available.
  • Linux: confirmed the browser opens via xdg-open when available.
• Verified that the HF token is no longer printed in the terminal when entered.