feat: add agent registry manifests and coverage

Introduce a manifest-backed agent registry surface and align graph tests with the new runtime prompt and tool indexing behavior.

backend/app/agents/registry/loader.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from app.agents.registry.builtins import (
+    BUILTIN_AGENT_MANIFESTS,
+    BUILTIN_CAPABILITY_MANIFESTS,
+    BUILTIN_SPECIALIST_TEMPLATE_MANIFESTS,
+    BUILTIN_SUB_COMMANDER_MANIFESTS,
+)
+from app.agents.registry.models import (
+    AgentManifest,
+    CapabilityManifest,
+    SpecialistTemplateManifest,
+    SubCommanderManifest,
+)
+
+
+@dataclass(frozen=True)
+class RegistryBundle:
+    agents: tuple[AgentManifest, ...]
+    sub_commanders: tuple[SubCommanderManifest, ...]
+    capabilities: tuple[CapabilityManifest, ...]
+    specialist_templates: tuple[SpecialistTemplateManifest, ...]
+
+
+def load_builtin_registry_bundle() -> RegistryBundle:
+    return RegistryBundle(
+        agents=BUILTIN_AGENT_MANIFESTS,
+        sub_commanders=BUILTIN_SUB_COMMANDER_MANIFESTS,
+        capabilities=BUILTIN_CAPABILITY_MANIFESTS,
+        specialist_templates=BUILTIN_SPECIALIST_TEMPLATE_MANIFESTS,
+    )