style: autoformat src and tests

Repository-wide formatting pass over src/ and tests/: sorted imports and
__all__ entries, dropped redundant `# -*- coding: utf-8 -*-` headers,
unquoted forward-reference annotations (safe under
`from __future__ import annotations`), merged nested `with` statements,
moved `Iterable` and friends from `typing` to `collections.abc`, and
removed `# noqa: F401` markers that no longer suppress anything.

No behavioural changes. Unit and integration suites show the same four
pre-existing failures before and after the pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/integration/test_branch_cli.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python3
-# -*- coding: utf-8 -*-
 
 """
 Integration tests for the `pkgmgr branch` CLI wiring.
@@ -16,8 +15,8 @@ from __future__ import annotations
 import unittest
 from unittest.mock import patch
 
-from pkgmgr.cli.parser import create_parser
 from pkgmgr.cli.commands.branch import handle_branch
+from pkgmgr.cli.parser import create_parser
 
 
 class TestBranchCLI(unittest.TestCase):