Initial commit: Python SDK for Dragonchain (Prime)

Synchronous Python SDK modeled on prime-sdk-go. Provides DC1-HMAC-SHA256
auth, dataclass models, and resource clients for system, transaction,
transaction-type, smart-contract, and block endpoints, plus a YAML
credentials loader.

prime_sdk/errors.py

@@ -0,0 +1,19 @@
+"""Exceptions raised by the Dragonchain SDK."""
+
+
+class DragonchainError(Exception):
+    """Base class for all errors raised by the SDK."""
+
+
+class DragonchainAPIError(DragonchainError):
+    """Raised when the Dragonchain node returns an HTTP error (status >= 400).
+
+    Attributes:
+        status_code: The HTTP status code returned by the node.
+        body: The (trimmed) response body returned by the node.
+    """
+
+    def __init__(self, status_code: int, body: str):
+        self.status_code = status_code
+        self.body = body
+        super().__init__(f"API error (status {status_code}): {body}")