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/models.py

@@ -0,0 +1,32 @@
+from pydantic import BaseModel
+
+
+class AgentManifest(BaseModel):
+    agent_id: str
+    display_name: str
+    role_value: str
+    system_prompt_key: str
+    routing_hints: list[str]
+    default_sub_commanders: list[str]
+    skill_context_key: str | None = None
+    continuity_policy: str | None = None
+    clarification_policy: str | None = None
+
+
+class SubCommanderManifest(BaseModel):
+    sub_commander_id: str
+    parent_agent_id: str
+    prompt_text: str
+    capability_ids: list[str]
+
+
+class CapabilityManifest(BaseModel):
+    capability_id: str
+    tool_name: str
+
+
+class SpecialistTemplateManifest(BaseModel):
+    template_id: str
+    display_name: str
+    description: str
+    allowed_capability_ids: list[str] | None = None