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/indexes.py

@@ -16,6 +16,7 @@ from app.agents.registry.models import (
 @dataclass(frozen=True)
 class RegistryIndexes:
     agent_by_id: Mapping[str, AgentManifest]
+    agent_by_role_value: Mapping[str, AgentManifest]
     sub_commander_by_id: Mapping[str, SubCommanderManifest]
     capability_by_id: Mapping[str, CapabilityManifest]
     specialist_template_by_id: Mapping[str, SpecialistTemplateManifest]
@@ -24,6 +25,7 @@ class RegistryIndexes:
     skill_context_key_by_agent_id: Mapping[str, str]
     capability_id_by_tool_name: Mapping[str, str]
     capability_ids_by_sub_commander_id: Mapping[str, tuple[str, ...]]
+    spawnable_role_values_by_agent_id: Mapping[str, tuple[str, ...]]
 
 
 def summarize_registry_indexes(indexes: RegistryIndexes) -> dict[str, int]:
@@ -50,6 +52,9 @@ def build_registry_indexes(bundle: RegistryBundle) -> RegistryIndexes:
 
     return RegistryIndexes(
         agent_by_id=MappingProxyType(agent_by_id),
+        agent_by_role_value=MappingProxyType({
+            agent.role_value: agent for agent in bundle.agents
+        }),
         sub_commander_by_id=MappingProxyType(sub_commander_by_id),
         capability_by_id=MappingProxyType(capability_by_id),
         specialist_template_by_id=MappingProxyType(specialist_template_by_id),
@@ -73,4 +78,9 @@ def build_registry_indexes(bundle: RegistryBundle) -> RegistryIndexes:
             sub_commander.sub_commander_id: tuple(sub_commander.capability_ids)
             for sub_commander in bundle.sub_commanders
         }),
+        spawnable_role_values_by_agent_id=MappingProxyType({
+            agent.agent_id: tuple(agent.allowed_spawn_role_values)
+            for agent in bundle.agents
+            if agent.can_spawn_children and agent.allowed_spawn_role_values
+        }),
     )