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/Notifier/StellaOps.Notifier/docs/incident-mode-rules.sample.json

@@ -0,0 +1,51 @@
+[
+  {
+    "ruleId": "incident-start-default",
+    "tenantId": "tenant-a",
+    "name": "Incident mode activated",
+    "enabled": true,
+    "match": {
+      "eventKinds": [
+        "incident.mode.start"
+      ]
+    },
+    "actions": [
+      {
+        "actionId": "incident-start-slack",
+        "channel": "slack-ops",
+        "template": "tmpl-incident-start",
+        "metadata": {
+          "trace_link": "{{payload.links.trace}}",
+          "evidence_link": "{{payload.links.evidence}}",
+          "retention_note": "Retention extended to {{payload.retentionDays}} days while incident mode is active.",
+          "quiet_hours_override": "true",
+          "legal_jurisdiction": "{{payload.legal.jurisdiction}}",
+          "legal_ticket": "{{payload.legal.ticket}}"
+        }
+      }
+    ]
+  },
+  {
+    "ruleId": "incident-stop-default",
+    "tenantId": "tenant-a",
+    "name": "Incident mode cleared",
+    "enabled": true,
+    "match": {
+      "eventKinds": [
+        "incident.mode.stop"
+      ]
+    },
+    "actions": [
+      {
+        "actionId": "incident-stop-email",
+        "channel": "email-compliance",
+        "template": "tmpl-incident-stop",
+        "metadata": {
+          "summary_link": "{{payload.links.timeline}}",
+          "retention_reset_note": "Retention reverts to baseline {{payload.retentionBaselineDays}} days.",
+          "legal_log_path": "{{payload.legal.logPath}}"
+        }
+      }
+    ]
+  }
+]