fix: use cluster SSH key config and simplify BACKENDAI environment handling

[inureyes]

Summary

• Fixed SSH key configuration to properly use cluster-specific SSH keys from config file
• Simplified BACKENDAI environment variable handling by removing complex cluster matching logic
• Changed auto-generated cluster name from 'backendai' to 'bai_auto' to avoid naming conflicts

Problem

1. When using bssh -c <cluster>, the SSH key specified in cluster configuration was ignored
2. BACKENDAI environment detection had unnecessarily complex logic for matching existing clusters
3. Auto-generated cluster name 'backendai' could conflict with user-defined clusters

Solution

1. SSH Key Resolution: Modified all command handlers to properly resolve SSH keys with priority:

   • CLI -i flag (highest priority)
   • Cluster-specific ssh_key from config
   • Default SSH key fallback

2. BACKENDAI Simplification: Removed complex cluster matching logic. Now when BACKENDAI environment variables are detected:

   • Simply creates a 'bai_auto' cluster with default settings
   • Uses port 2200 and current username
   • Falls back to standard SSH key locations

3. Naming: Changed auto-generated cluster name to 'bai_auto' to clearly indicate it's automatically generated and avoid conflicts

Test Results

• ✅ bssh -c orin ls now correctly uses ~/.ssh/id_rsa_orin from config
• ✅ BACKENDAI environment creates independent 'bai_auto' cluster
• ✅ Path expansion (~) works correctly for SSH keys
• ✅ Users can still create their own 'backendai' cluster without conflicts