feat(lib): add errors module

Introduce AiMouseError base class plus ModelLoadError and
GenerationError subclasses so downstream consumers can catch the
umbrella or specific failure modes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/unit/test_errors.py

@@ -0,0 +1,19 @@
+"""Test the error hierarchy."""
+from __future__ import annotations
+
+import pytest
+
+from ai_mouse import errors
+
+
+def test_model_load_error_is_aimouse_error() -> None:
+    assert issubclass(errors.ModelLoadError, errors.AiMouseError)
+
+
+def test_generation_error_is_aimouse_error() -> None:
+    assert issubclass(errors.GenerationError, errors.AiMouseError)
+
+
+def test_can_catch_specific_with_general() -> None:
+    with pytest.raises(errors.AiMouseError):
+        raise errors.ModelLoadError("test")