feat: Add in-memory implementations for issuer audit, key, repository, and trust management

- Introduced InMemoryIssuerAuditSink to retain audit entries for testing.
- Implemented InMemoryIssuerKeyRepository for deterministic key storage.
- Created InMemoryIssuerRepository to manage issuer records in memory.
- Added InMemoryIssuerTrustRepository for managing issuer trust overrides.
- Each repository utilizes concurrent collections for thread-safe operations.
- Enhanced deprecation tracking with a comprehensive YAML schema for API governance.

.spectral.yaml

@@ -165,3 +165,69 @@ rules:
               in:
                 const: header
             required: [name, in]
+
+  # --- Deprecation Metadata Rules (per APIGOV-63-001) ---
+
+  stella-deprecated-has-metadata:
+    description: "Deprecated operations must have x-deprecation extension with required fields"
+    message: "Add x-deprecation metadata (deprecatedAt, sunsetAt, successorPath, reason) to deprecated operations"
+    given: "$.paths[*][*][?(@.deprecated == true)]"
+    severity: error
+    then:
+      field: x-deprecation
+      function: schema
+      functionOptions:
+        schema:
+          type: object
+          required:
+            - deprecatedAt
+            - sunsetAt
+            - successorPath
+            - reason
+          properties:
+            deprecatedAt:
+              type: string
+              format: date-time
+            sunsetAt:
+              type: string
+              format: date-time
+            successorPath:
+              type: string
+            successorOperationId:
+              type: string
+            reason:
+              type: string
+            migrationGuide:
+              type: string
+              format: uri
+            notificationChannels:
+              type: array
+              items:
+                type: string
+                enum: [slack, teams, email, webhook]
+
+  stella-deprecated-sunset-future:
+    description: "Sunset dates should be in the future (warn if sunset already passed)"
+    message: "x-deprecation.sunsetAt should be a future date"
+    given: "$.paths[*][*].x-deprecation.sunsetAt"
+    severity: warn
+    then:
+      function: truthy
+
+  stella-deprecated-migration-guide:
+    description: "Deprecated operations should include a migration guide URL"
+    message: "Consider adding x-deprecation.migrationGuide for consumer guidance"
+    given: "$.paths[*][*][?(@.deprecated == true)].x-deprecation"
+    severity: hint
+    then:
+      field: migrationGuide
+      function: truthy
+
+  stella-deprecated-notification-channels:
+    description: "Deprecated operations should specify notification channels"
+    message: "Add x-deprecation.notificationChannels to enable deprecation notifications"
+    given: "$.paths[*][*][?(@.deprecated == true)].x-deprecation"
+    severity: hint
+    then:
+      field: notificationChannels
+      function: truthy