feat(api): add MS23 mission control Prisma schema

Add 4 new models for Mission Control:
- AgentConversationMessage: per-agent message storage with SSE streaming
- AgentSessionTree: parent/child subagent relationships (taskSource field for mosaic-queue integration)
- AgentProviderConfig: external provider registration per workspace (encrypted credentials)
- OperatorAuditLog: append-only audit trail for barge-in/kill/pause actions

Closes #693 (partial — schema only)

apps/api/prisma/schema.prisma

@@ -1739,3 +1739,66 @@ model UserAgent {
   @@unique([userId, name])
   @@index([userId])
 }
+
+// MS23: Agent conversation messages for Mission Control streaming
+model AgentConversationMessage {
+  id        String   @id @default(cuid())
+  sessionId String
+  provider  String   @default("internal")
+  role      String
+  content   String
+  timestamp DateTime @default(now())
+  metadata  Json     @default("{}")
+
+  @@index([sessionId, timestamp])
+}
+
+// MS23: Agent session tree for parent/child relationships
+model AgentSessionTree {
+  id              String   @id @default(cuid())
+  sessionId       String   @unique
+  parentSessionId String?
+  provider        String   @default("internal")
+  missionId       String?
+  taskId          String?
+  taskSource      String?  @default("internal")
+  agentType       String?
+  status          String   @default("spawning")
+  spawnedAt       DateTime @default(now())
+  completedAt     DateTime?
+  metadata        Json     @default("{}")
+
+  @@index([parentSessionId])
+  @@index([missionId])
+}
+
+// MS23: External agent provider configuration per workspace
+model AgentProviderConfig {
+  id          String   @id @default(cuid())
+  workspaceId String
+  name        String
+  provider    String
+  gatewayUrl  String
+  credentials Json     @default("{}")
+  isActive    Boolean  @default(true)
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+
+  @@unique([workspaceId, name])
+}
+
+// MS23: Audit log for operator interventions
+model OperatorAuditLog {
+  id        String   @id @default(cuid())
+  userId    String
+  sessionId String
+  provider  String
+  action    String
+  content   String?
+  metadata  Json     @default("{}")
+  createdAt DateTime @default(now())
+
+  @@index([sessionId])
+  @@index([userId])
+  @@index([createdAt])
+}