feat(core): add systemd unit collector

src/Vantage.Core/Collectors/Systemd/ISystemdProvider.cs

@@ -0,0 +1,8 @@
+using Vantage.Core.Models;
+
+namespace Vantage.Core.Collectors.Systemd;
+
+public interface ISystemdProvider
+{
+    Task<IReadOnlyList<SystemdUnit>> GetServicesAsync(HostConfig host, CancellationToken cancellationToken);
+}

src/Vantage.Core/Collectors/Systemd/MockSystemdProvider.cs

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+using Vantage.Core.Models;
+
+namespace Vantage.Core.Collectors.Systemd;
+
+public sealed class MockSystemdProvider : ISystemdProvider
+{
+    private static readonly SystemdUnit[] Units =
+    {
+        new("docker.service", "loaded", "active", "running", "Docker Application Container Engine"),
+        new("nginx.service", "loaded", "active", "running", "A high performance web server"),
+        new("postgresql.service", "loaded", "active", "running", "PostgreSQL RDBMS"),
+        new("ssh.service", "loaded", "active", "running", "OpenBSD Secure Shell server"),
+        new("prometheus.service", "loaded", "active", "running", "Prometheus monitoring"),
+        new("fail2ban.service", "loaded", "active", "running", "Fail2Ban Service"),
+        new("systemd-journald.service", "loaded", "active", "running", "Journal Service"),
+        new("backup.service", "loaded", "failed", "failed", "Nightly backup job"),
+        new("certbot-renew.service", "loaded", "inactive", "dead", "Certbot renewal")
+    };
+
+    public Task<IReadOnlyList<SystemdUnit>> GetServicesAsync(HostConfig host, CancellationToken cancellationToken) =>
+        Task.FromResult<IReadOnlyList<SystemdUnit>>(Units);
+}

src/Vantage.Core/Collectors/Systemd/SshSystemdProvider.cs

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+using Vantage.Core.Models;
+using Vantage.Core.Ssh;
+
+namespace Vantage.Core.Collectors.Systemd;
+
+public sealed class SshSystemdProvider : ISystemdProvider
+{
+    public const string Command =
+        "systemctl list-units --type=service --all --no-legend --no-pager --plain";
+
+    private readonly ISshCommandRunner runner;
+
+    public SshSystemdProvider(ISshCommandRunner runner)
+    {
+        this.runner = runner;
+    }
+
+    public async Task<IReadOnlyList<SystemdUnit>> GetServicesAsync(HostConfig host, CancellationToken cancellationToken)
+    {
+        var output = await runner.RunAsync(host, Command, cancellationToken).ConfigureAwait(false);
+        return SystemdUnitParser.Parse(output);
+    }
+}

src/Vantage.Core/Collectors/Systemd/SystemdUnit.cs

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+namespace Vantage.Core.Collectors.Systemd;
+
+public readonly record struct SystemdUnit(
+    string Unit,
+    string Load,
+    string Active,
+    string Sub,
+    string Description)
+{
+    public bool IsRunning => Active == "active" && Sub == "running";
+    public bool IsFailed => Active == "failed" || Sub == "failed";
+}

src/Vantage.Core/Collectors/Systemd/SystemdUnitParser.cs

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+namespace Vantage.Core.Collectors.Systemd;
+
+public static class SystemdUnitParser
+{
+    public static IReadOnlyList<SystemdUnit> Parse(string listUnitsOutput)
+    {
+        var units = new List<SystemdUnit>();
+
+        foreach (var rawLine in listUnitsOutput.Replace("\r\n", "\n").Split('\n'))
+        {
+            var line = StripLeadingMarker(rawLine).TrimEnd();
+            if (line.Length == 0)
+                continue;
+            if (!line.Contains(".service", StringComparison.Ordinal))
+                continue;
+
+            var fields = line.Split(' ', 5, StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+            if (fields.Length < 4)
+                continue;
+
+            units.Add(new SystemdUnit(
+                Unit: fields[0],
+                Load: fields[1],
+                Active: fields[2],
+                Sub: fields[3],
+                Description: fields.Length >= 5 ? fields[4] : ""));
+        }
+
+        return units;
+    }
+
+    private static string StripLeadingMarker(string line)
+    {
+        var index = 0;
+        while (index < line.Length && !IsUnitStart(line[index]))
+            index++;
+        return line[index..];
+    }
+
+    private static bool IsUnitStart(char c) => char.IsLetterOrDigit(c) && c < 128;
+}