Replace Go SSH library with native ssh command

Major rewrite to use native ssh command instead of Go SSH library:

BREAKING CHANGE: Now requires ssh command in PATH

Benefits:
- Full SSH feature support including ProxyCommand
- Works with SSH containers and jump hosts
- Supports all SSH authentication methods
- Consistent behavior with terminal SSH
- No more custom SSH client implementation

Changes:
- Port detection now uses 'ssh hostname command'
- Port forwarding uses 'ssh -L localport:localhost:remoteport hostname'
- Connection testing uses native ssh command
- Removed golang.org/x/crypto/ssh dependency
- Updated documentation to reflect SSH compatibility

This fixes issues with SSH containers that require ProxyCommand
and provides full compatibility with user SSH configurations.

Co-authored-by: Ona <no-reply@ona.com>

ssh_config.go

@@ -175,11 +175,15 @@ func (sc *SSHConfig) GetHosts() []SSHHost {
 	return sc.Hosts
 }
 
-// GetHostByName returns a specific host by name
+// GetHostByName returns a specific host by name with expanded variables
 func (sc *SSHConfig) GetHostByName(name string) (*SSHHost, error) {
 	for _, host := range sc.Hosts {
 		if host.Name == name {
-			return &host, nil
+			// Return a copy with expanded shell variables
+			expandedHost := host
+			expandedHost.User = expandShellVars(host.User)
+			expandedHost.Identity = expandShellVars(host.Identity)
+			return &expandedHost, nil
 		}
 	}
 	return nil, fmt.Errorf("host '%s' not found", name)