chore: add MyPy type checking config with baseline ignore rules

Adds [tool.mypy] section to pyproject.toml with permissive baseline settings
and disables error codes that fire against the existing untyped codebase.
Adds the mirrors-mypy pre-commit hook (v1.19.1) running over python/ at once.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.pre-commit-config.yaml

@@ -35,3 +35,12 @@ repos:
       - id: flake8
         files: ^python/
         args: [--config=.flake8]
+
+  - repo: https://github.com/pre-commit/mirrors-mypy
+    rev: "v1.19.1"
+    hooks:
+      - id: mypy
+        files: ^python/
+        args: [--config-file=pyproject.toml]
+        pass_filenames: false
+        additional_dependencies: []