Track 1: Add custom exception hierarchy

[nielsweistra]

Track 1: Add custom exception hierarchy

Overview

All error paths in itl-policy-builder currently raise bare ValueError or
RuntimeError. This makes it impossible for callers to catch errors at the right
level of specificity without catching all Exceptions.

Goal

Introduce a typed exception hierarchy rooted at PolicyBuilderError:

class PolicyBuilderError(Exception): ...
class PolicyValidationError(PolicyBuilderError): ...
class PolicyEvaluationError(PolicyBuilderError): ...
class PolicyDeployError(PolicyBuilderError): ...
class PolicyExportError(PolicyBuilderError): ...

Implementation Steps

1. Create src/itl_policy_builder/exceptions.py with the hierarchy above
2. Replace all raise ValueError(...) / raise RuntimeError(...) with the appropriate typed exception
3. Export all exception classes from src/itl_policy_builder/__init__.py
4. Add unit tests in tests/unit/test_exceptions.py

Acceptance Criteria

• PolicyBuilderError is the base for all package-specific exceptions
• All ValueError / RuntimeError in library code replaced with typed exceptions
• Exception classes exported from package top-level
• Unit tests covering each exception type
• No breaking changes to existing public API signatures