feat: add agent visibility APIs and harden runtime verification

Add Day 4 visibility endpoints and response models, strengthen collaboration/task verification behavior, and patch conversation schema startup migration for agent_state compatibility. Extend backend regression coverage for runtime schemas, verifier behavior, visibility APIs, router auth, and legacy conversation list loading.

backend/app/agents/schemas/__init__.py

@@ -1,10 +1,25 @@
 from app.agents.schemas.event import AgentEvent
-from app.agents.schemas.task import AgentTask, TaskResult, TaskLifecycleStatus, VerificationStatus
+from app.agents.schemas.message import AgentMessage
+from app.agents.schemas.task import (
+    AgentTask,
+    CollaborationBudget,
+    InterruptRecord,
+    RecoveryRecord,
+    TaskLifecycleStatus,
+    TaskResult,
+    TaskResultStatus,
+    VerificationStatus,
+)
 
 __all__ = [
     "AgentEvent",
+    "AgentMessage",
     "AgentTask",
+    "CollaborationBudget",
+    "InterruptRecord",
+    "RecoveryRecord",
     "TaskLifecycleStatus",
     "TaskResult",
+    "TaskResultStatus",
     "VerificationStatus",
 ]