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}";
+    }
+}