feat: Implement MongoDB orchestrator storage with registry, commands, and heartbeats

- Added NullAdvisoryObservationEventTransport for handling advisory observation events.
- Created IOrchestratorRegistryStore interface for orchestrator registry operations.
- Implemented MongoOrchestratorRegistryStore for MongoDB interactions with orchestrator data.
- Defined OrchestratorCommandDocument and OrchestratorCommandRecord for command handling.
- Added OrchestratorHeartbeatDocument and OrchestratorHeartbeatRecord for heartbeat tracking.
- Created OrchestratorRegistryDocument and OrchestratorRegistryRecord for registry management.
- Developed tests for orchestrator collections migration and MongoOrchestratorRegistryStore functionality.
- Introduced AirgapImportRequest and AirgapImportValidator for air-gapped VEX bundle imports.
- Added incident mode rules sample JSON for notifier configuration.

src/Notify/__Libraries/StellaOps.Notify.Storage.Mongo/Migrations/EnsurePackApprovalsCollectionMigration.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using Microsoft.Extensions.Logging;
 using StellaOps.Notify.Storage.Mongo.Internal;
 
@@ -22,8 +23,13 @@ internal sealed class EnsurePackApprovalsCollectionMigration : INotifyMongoMigra
             .ListCollectionNamesAsync(cancellationToken: cancellationToken)
             .ConfigureAwait(false);
 
-        var existing = await cursor.ToListAsync(cancellationToken).ConfigureAwait(false);
-        if (existing.Contains(target, StringComparer.Ordinal))
+        var existing = new HashSet<string>(StringComparer.Ordinal);
+        while (await cursor.MoveNextAsync(cancellationToken).ConfigureAwait(false))
+        {
+            existing.UnionWith(cursor.Current);
+        }
+
+        if (existing.Contains(target))
         {
             return;
         }

src/Notify/__Libraries/StellaOps.Notify.Storage.Mongo/Options/NotifyMongoOptions.cs

@@ -16,9 +16,11 @@ public sealed class NotifyMongoOptions
 
     public string DeliveriesCollection { get; set; } = "deliveries";
 
-    public string DigestsCollection { get; set; } = "digests";
-
-    public string LocksCollection { get; set; } = "locks";
+    public string DigestsCollection { get; set; } = "digests";
+
+    public string PackApprovalsCollection { get; set; } = "pack_approvals";
+
+    public string LocksCollection { get; set; } = "locks";
 
     public string AuditCollection { get; set; } = "audit";