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/builtins.py

@@ -57,6 +57,19 @@ TOP_LEVEL_AGENT_ROUTING_HINTS: dict[str, tuple[str, ...]] = {
     ),
 }
 
+TOP_LEVEL_AGENT_ALLOWED_SPAWN_ROLES: dict[str, tuple[str, ...]] = {
+    AgentRole.MASTER.value: (
+        AgentRole.SCHEDULE_PLANNER.value,
+        AgentRole.EXECUTOR.value,
+        AgentRole.LIBRARIAN.value,
+        AgentRole.ANALYST.value,
+    ),
+    AgentRole.SCHEDULE_PLANNER.value: (AgentRole.SCHEDULE_PLANNER.value,),
+    AgentRole.EXECUTOR.value: (AgentRole.EXECUTOR.value,),
+    AgentRole.LIBRARIAN.value: (AgentRole.LIBRARIAN.value,),
+    AgentRole.ANALYST.value: (AgentRole.ANALYST.value,),
+}
+
 SUB_COMMANDER_PARENT_AGENT_IDS: dict[str, str] = {
     "schedule_analysis": AgentRole.SCHEDULE_PLANNER.value,
     "schedule_planning": AgentRole.SCHEDULE_PLANNER.value,
@@ -77,6 +90,8 @@ BUILTIN_AGENT_MANIFESTS: tuple[AgentManifest, ...] = tuple(
         system_prompt_key=role.value,
         routing_hints=list(TOP_LEVEL_AGENT_ROUTING_HINTS[role.value]),
         default_sub_commanders=list(TOP_LEVEL_AGENT_DEFAULT_SUB_COMMANDERS[role.value]),
+        can_spawn_children=bool(TOP_LEVEL_AGENT_ALLOWED_SPAWN_ROLES[role.value]),
+        allowed_spawn_role_values=list(TOP_LEVEL_AGENT_ALLOWED_SPAWN_ROLES[role.value]),
         skill_context_key=role.value.replace("agent_", ""),
     )
     for role in AgentRole