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/e2e/test_nix_build_pkgmgr.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python3
-# -*- coding: utf-8 -*-
 
 """
 E2E test to inspect the Nix environment and build the pkgmgr flake
@@ -22,7 +21,6 @@ import os
 import subprocess
 import unittest
 
-
 # Resolve project root (the repo where flake.nix lives, e.g. /src)
 PROJECT_ROOT = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", ".."))