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/registry/models.py

@@ -1,6 +1,6 @@
 from enum import Enum
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 
 class PermissionClass(str, Enum):
@@ -23,6 +23,8 @@ class AgentManifest(BaseModel):
     system_prompt_key: str
     routing_hints: list[str]
     default_sub_commanders: list[str]
+    can_spawn_children: bool = False
+    allowed_spawn_role_values: list[str] = Field(default_factory=list)
     skill_context_key: str | None = None
     continuity_policy: str | None = None
     clarification_policy: str | None = None