feat: add secure agent enrollment

backend/app/agent_auth/exceptions.py

@@ -0,0 +1,21 @@
+"""Domain errors for Agent authentication."""
+
+
+class AgentAuthenticationError(Exception):
+    """Base Agent authentication error."""
+
+
+class AgentAuthenticationUnavailable(AgentAuthenticationError):
+    """Agent authentication cannot currently be performed."""
+
+
+class AgentAuthenticationFailed(AgentAuthenticationError):
+    """The supplied Agent secret is invalid."""
+
+
+class AgentIdentityUnauthorized(AgentAuthenticationError):
+    """The supplied Agent identity is not authorized."""
+
+
+class AgentCredentialRevoked(AgentAuthenticationError):
+    """The Agent's permanent credential has been revoked."""