feat: add AdditionalConfig parameter and guest agent cmdlets (closes #2, #3)

Issue #3 — Set-PveVmConfig / Set-PveContainerConfig:
- Add -AdditionalConfig [Hashtable] for arbitrary PVE config keys
  (scsi0, boot, agent, net0, ide2, cicustom, etc.)
- Add -Delete [string] for removing config keys
- Unit tests for both new parameters

Issue #2 — QEMU Guest Agent cmdlets:
- Test-PveVmGuestAgent: pings guest agent, returns bool
- Get-PveVmGuestNetwork: returns guest network interfaces with IPs
- Invoke-PveVmGuestExec: executes command in guest, returns output
- New models: PveGuestNetworkInterface, PveGuestIpAddress
- Service methods in VmService for all agent operations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/PSProxmoxVE.Core/Models/Vms/PveGuestIpAddress.cs

@@ -0,0 +1,27 @@
+using System.Text.Json.Serialization;
+using Newtonsoft.Json;
+
+namespace PSProxmoxVE.Core.Models.Vms;
+
+/// <summary>
+/// Represents an IP address reported by the QEMU guest agent.
+/// </summary>
+public class PveGuestIpAddress
+{
+    /// <summary>The IP address string (e.g., "192.168.1.100" or "fe80::1").</summary>
+    [JsonPropertyName("ip-address")]
+    [JsonProperty("ip-address")]
+    public string Address { get; set; } = string.Empty;
+
+    /// <summary>The address type: "ipv4" or "ipv6".</summary>
+    [JsonPropertyName("ip-address-type")]
+    [JsonProperty("ip-address-type")]
+    public string Type { get; set; } = string.Empty;
+
+    /// <summary>The network prefix length (e.g., 24 for a /24 subnet).</summary>
+    [JsonPropertyName("prefix")]
+    [JsonProperty("prefix")]
+    public int Prefix { get; set; }
+
+    public override string ToString() => $"{Address}/{Prefix} ({Type})";
+}

src/PSProxmoxVE.Core/Models/Vms/PveGuestNetworkInterface.cs

@@ -0,0 +1,33 @@
+using System.Text.Json.Serialization;
+using Newtonsoft.Json;
+
+namespace PSProxmoxVE.Core.Models.Vms;
+
+/// <summary>
+/// Represents a network interface reported by the QEMU guest agent.
+/// </summary>
+public class PveGuestNetworkInterface
+{
+    /// <summary>The interface name (e.g., "eth0", "ens18", "lo").</summary>
+    [JsonPropertyName("name")]
+    [JsonProperty("name")]
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>The hardware (MAC) address of the interface.</summary>
+    [JsonPropertyName("hardware-address")]
+    [JsonProperty("hardware-address")]
+    public string? HardwareAddress { get; set; }
+
+    /// <summary>The IP addresses assigned to this interface.</summary>
+    [JsonPropertyName("ip-addresses")]
+    [JsonProperty("ip-addresses")]
+    public PveGuestIpAddress[] IpAddresses { get; set; } = System.Array.Empty<PveGuestIpAddress>();
+
+    public override string ToString()
+    {
+        var ips = IpAddresses.Length > 0
+            ? string.Join(", ", System.Array.ConvertAll(IpAddresses, ip => ip.ToString()))
+            : "no addresses";
+        return $"{Name} ({HardwareAddress ?? "N/A"}): {ips}";
+    }
+}

src/PSProxmoxVE.Core/Services/VmService.cs

@@ -353,5 +353,89 @@ namespace PSProxmoxVE.Core.Services
             task.Node = node;
             return task;
         }
+
+        // -------------------------------------------------------------------------
+        // QEMU Guest Agent
+        // -------------------------------------------------------------------------
+
+        /// <summary>
+        /// Pings the QEMU guest agent on the specified VM. Returns true if responsive.
+        /// </summary>
+        public bool PingGuestAgent(PveSession session, string node, int vmid)
+        {
+            if (session == null) throw new ArgumentNullException(nameof(session));
+            if (string.IsNullOrWhiteSpace(node)) throw new ArgumentNullException(nameof(node));
+
+            using var client = new PveHttpClient(session);
+            try
+            {
+                client.PostAsync($"nodes/{node}/qemu/{vmid}/agent/ping").GetAwaiter().GetResult();
+                return true;
+            }
+            catch
+            {
+                return false;
+            }
+        }
+
+        /// <summary>
+        /// Retrieves network interface information from the QEMU guest agent.
+        /// </summary>
+        public PveGuestNetworkInterface[] GetGuestNetworkInterfaces(PveSession session, string node, int vmid)
+        {
+            if (session == null) throw new ArgumentNullException(nameof(session));
+            if (string.IsNullOrWhiteSpace(node)) throw new ArgumentNullException(nameof(node));
+
+            using var client = new PveHttpClient(session);
+            var response = client.GetAsync($"nodes/{node}/qemu/{vmid}/agent/network-get-interfaces")
+                .GetAwaiter().GetResult();
+            var data = JObject.Parse(response)["data"];
+            var result = data?["result"];
+            return result?.ToObject<PveGuestNetworkInterface[]>() ?? Array.Empty<PveGuestNetworkInterface>();
+        }
+
+        /// <summary>
+        /// Executes a command inside the guest via the QEMU guest agent.
+        /// Returns the PID of the spawned process.
+        /// </summary>
+        public int ExecuteGuestCommand(PveSession session, string node, int vmid,
+            string command, string[]? args = null)
+        {
+            if (session == null) throw new ArgumentNullException(nameof(session));
+            if (string.IsNullOrWhiteSpace(node)) throw new ArgumentNullException(nameof(node));
+            if (string.IsNullOrWhiteSpace(command)) throw new ArgumentNullException(nameof(command));
+
+            using var client = new PveHttpClient(session);
+            var data = new Dictionary<string, string>
+            {
+                ["command"] = command
+            };
+
+            if (args != null && args.Length > 0)
+            {
+                // PVE expects input-data for arguments passed as a JSON-encoded string array
+                var argsJson = Newtonsoft.Json.JsonConvert.SerializeObject(args);
+                data["input-data"] = argsJson;
+            }
+
+            var response = client.PostAsync($"nodes/{node}/qemu/{vmid}/agent/exec", data)
+                .GetAwaiter().GetResult();
+            var pid = JObject.Parse(response)["data"]?["pid"]?.ToObject<int>() ?? 0;
+            return pid;
+        }
+
+        /// <summary>
+        /// Gets the status/result of a guest agent exec command by PID.
+        /// </summary>
+        public JObject GetGuestExecStatus(PveSession session, string node, int vmid, int pid)
+        {
+            if (session == null) throw new ArgumentNullException(nameof(session));
+            if (string.IsNullOrWhiteSpace(node)) throw new ArgumentNullException(nameof(node));
+
+            using var client = new PveHttpClient(session);
+            var response = client.GetAsync($"nodes/{node}/qemu/{vmid}/agent/exec-status?pid={pid}")
+                .GetAwaiter().GetResult();
+            return JObject.Parse(response)["data"] as JObject ?? new JObject();
+        }
     }
 }