fix(config-writer): SIGHUP via libc kill(2) instead of /bin/kill

The aspnet:9.0 runtime base image doesn't ship procps, so
Process.Start(\"kill\", \"-HUP <pid>\") failed with ENOENT — the first
reconcile after boot wrote the new nats.conf to disk, then threw when
signalling, and subsequent reconciles saw the on-disk text already
matched the target and skipped the retry. Net effect: nats-server kept
running with whatever config it parsed at startup, and configuration
changes never took effect at runtime.

Switch to a direct P/Invoke to libc kill(2) so we don't depend on any
external binary.

src/VolcanicCore.Core/NatsConfig/ConfigWriter.cs

@@ -1,5 +1,5 @@
-using System.Diagnostics;
 using System.Net.Http;
+using System.Runtime.InteropServices;
 using Microsoft.Extensions.Logging;
 
 namespace VolcanicCore.Core.NatsConfig;
@@ -69,27 +69,26 @@ public sealed class ConfigWriter : IConfigWriter
         return true;
     }
 
+    // POSIX kill(2). Signalling via libc means we don't depend on /bin/kill
+    // being present — the aspnet:9.0 base image ships without procps, so
+    // Process.Start("kill", ...) fails at runtime with ENOENT and the config
+    // reload silently never happens.
+    [DllImport("libc", SetLastError = true, EntryPoint = "kill")]
+    private static extern int LibcKill(int pid, int sig);
+
+    private const int SIGHUP = 1;
+
     private static void SendSighup(int processId)
     {
-        // Cross-platform: use kill(1) on Linux; on Windows nats-server doesn't support hot reload.
-        if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
-        {
-            using var proc = new Process
-            {
-                StartInfo = new ProcessStartInfo("kill", $"-HUP {processId}")
-                {
-                    UseShellExecute = false,
-                    RedirectStandardError = true,
-                    RedirectStandardOutput = true,
-                },
-            };
-            proc.Start();
-            proc.WaitForExit(2000);
-        }
-        else
-        {
+        if (!OperatingSystem.IsLinux() && !OperatingSystem.IsMacOS())
             throw new PlatformNotSupportedException(
                 "nats-server hot reload requires SIGHUP; Windows is not supported.");
+        var rc = LibcKill(processId, SIGHUP);
+        if (rc != 0)
+        {
+            var errno = Marshal.GetLastPInvokeError();
+            throw new InvalidOperationException(
+                $"kill(pid={processId}, SIGHUP) failed with errno={errno}");
         }
     }