style: sort Python imports with isort

Add isort configuration (profile=black, line_length=100) to pyproject.toml,
add isort pre-commit hook, and auto-sort imports across all Python source files.

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

.pre-commit-config.yaml

@@ -16,3 +16,9 @@ repos:
       - id: black
         language_version: python3
         args: [--config=pyproject.toml]
+
+  - repo: https://github.com/pycqa/isort
+    rev: 8.0.1
+    hooks:
+      - id: isort
+        args: [--settings-path=pyproject.toml]