Fix Pydantic model forward references and improve error messages

Fix issue where real-world pins.lock files were incorrectly reported as
malformed due to unresolved forward references in Pydantic models.

Changes:
- Call model_rebuild() on Package and LockFile after importing all
  package types to resolve forward references
- Chain validation errors using 'from e' to preserve detailed error
  information for debugging while showing user-friendly message

The model_rebuild() calls are necessary because the lockfile models use
forward references to package types (GAPackageDef, QuadPackageDef, etc.)
that are only imported later in the __init__.py file.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: robtaylor

chipflow/packaging/__init__.py

@@ -81,6 +81,11 @@
     PinCommand,
 )
 
+# Rebuild Pydantic models now that all package types are imported
+# This resolves forward references in the lockfile models
+Package.model_rebuild()
+LockFile.model_rebuild()
+
 # NOTE: This module is currently internal to the chipflow CLI.
 # The public API will be designed in a future PR after working through
 # real-world custom package examples.

chipflow/packaging/utils.py

@@ -36,11 +36,11 @@ def load_pinlock() -> LockFile:
         try:
             json = lockfile.read_text()
             return LockFile.model_validate_json(json)
-        except (pydantic.ValidationError, pydantic.PydanticUserError):
+        except (pydantic.ValidationError, pydantic.PydanticUserError) as e:
             raise ChipFlowError(
                 "Lockfile `pins.lock` is misformed. "
                 "Please remove and rerun `chipflow pin lock`"
-            )
+            ) from e
 
     raise ChipFlowError("Lockfile `pins.lock` not found. Run `chipflow pin lock`")