feat(core): add SSH command runner abstraction

src/Vantage.Core/Ssh/ISshCommandRunner.cs

@@ -0,0 +1,8 @@
+using Vantage.Core.Models;
+
+namespace Vantage.Core.Ssh;
+
+public interface ISshCommandRunner
+{
+    Task<string> RunAsync(HostConfig host, string command, CancellationToken cancellationToken);
+}

src/Vantage.Core/Ssh/ShellArgs.cs

@@ -0,0 +1,6 @@
+namespace Vantage.Core.Ssh;
+
+public static class ShellArgs
+{
+    public static string Quote(string value) => "'" + value.Replace("'", "'\\''") + "'";
+}

src/Vantage.Core/Ssh/SshNetCommandRunner.cs

@@ -0,0 +1,58 @@
+using Renci.SshNet;
+using Renci.SshNet.Common;
+using Vantage.Core.Models;
+
+namespace Vantage.Core.Ssh;
+
+public sealed class SshNetCommandRunner : ISshCommandRunner
+{
+    public async Task<string> RunAsync(HostConfig host, string command, CancellationToken cancellationToken)
+    {
+        var connectionInfo = BuildConnectionInfo(host);
+
+        using var client = new SshClient(connectionInfo);
+        await Task.Run(() => client.Connect(), cancellationToken).ConfigureAwait(false);
+        try
+        {
+            using var sshCommand = client.CreateCommand(command);
+            var output = await Task.Factory
+                .FromAsync(sshCommand.BeginExecute(), sshCommand.EndExecute)
+                .ConfigureAwait(false);
+
+            if (sshCommand.ExitStatus != 0 && !string.IsNullOrEmpty(sshCommand.Error))
+                throw new SshException($"Remote command failed (exit {sshCommand.ExitStatus}): {sshCommand.Error.Trim()}");
+
+            return output;
+        }
+        finally
+        {
+            client.Disconnect();
+        }
+    }
+
+    private static ConnectionInfo BuildConnectionInfo(HostConfig host)
+    {
+        var authentication = host.AuthMethod switch
+        {
+            SshAuthMethod.PrivateKey => PrivateKeyAuth(host),
+            SshAuthMethod.Password => new PasswordAuthenticationMethod(host.Username, host.Password ?? ""),
+            SshAuthMethod.Agent => throw new NotSupportedException(
+                "SSH agent authentication is not wired yet; configure PrivateKey or Password."),
+            _ => throw new ArgumentOutOfRangeException(nameof(host))
+        };
+
+        return new ConnectionInfo(host.Address, host.Port, host.Username, authentication);
+    }
+
+    private static AuthenticationMethod PrivateKeyAuth(HostConfig host)
+    {
+        if (string.IsNullOrWhiteSpace(host.PrivateKeyPath))
+            throw new InvalidOperationException($"Host '{host.Name}' uses PrivateKey auth but no key path is set.");
+
+        var keyFile = string.IsNullOrEmpty(host.PrivateKeyPassphrase)
+            ? new PrivateKeyFile(host.PrivateKeyPath)
+            : new PrivateKeyFile(host.PrivateKeyPath, host.PrivateKeyPassphrase);
+
+        return new PrivateKeyAuthenticationMethod(host.Username, keyFile);
+    }
+}