[Feature] Add NixOS module for running gateway as isolated system user

[jeanlucthumm]

Summary

The current module only supports home-manager (user-level systemd service). This means the gateway runs with full access to the user's personal files, SSH keys, credentials, etc.

Upstream issue: moltbot/moltbot#2341

Proposal

Add a NixOS module (nixosModules.clawdbot) that:

1. Creates a dedicated clawdbot system user with minimal privileges
2. Runs the gateway as a system-level systemd service
3. Applies systemd hardening options:
   • DynamicUser=true or dedicated user
   • ProtectHome=true
   • PrivateTmp=true
   • NoNewPrivileges=true
   • ProtectSystem=strict
   • etc.
4. Manages credentials in the isolated user's home

Use Case

Security-conscious users who want to run clawdbot on a server without exposing their personal files to the LLM.

Notes

• Claude OAuth credentials would need to be set up separately for the clawdbot user
• Could coexist with the home-manager module for different use cases