feat: scaffold P_Rec_Annot .NET 8 architecture

Plant3D2026/P_Rec_Annot/src/PRecAnnot.Telemetry/JsonlUsageSink.cs

@@ -0,0 +1,14 @@
+using System.Text.Json;
+using PRecAnnot.Abstractions;
+
+namespace PRecAnnot.Telemetry;
+
+public sealed class JsonlUsageSink(string path) : ICommandUsageSink
+{
+    private readonly object _gate = new();
+    public void Record(CommandUsageEvent usageEvent)
+    {
+        var json = JsonSerializer.Serialize(usageEvent);
+        lock (_gate) File.AppendAllText(path, json + Environment.NewLine);
+    }
+}