style: modernise typing and clean up lint findings

Repository-wide mechanical cleanup so `ruff check src tests` has a chance
of passing; no behavioural changes.

- Add `from __future__ import annotations` where PEP 604 unions are used.
  This has to come first: pyproject declares requires-python >= 3.9, where
  `X | None` is not evaluable at runtime unless annotations are stringified.
- Replace typing.List/Dict/Tuple/Set with the builtin generics and
  Optional[X] with X | None, then drop the imports that became unused.
  The four actions/*/__init__.py files needed this by hand because ruff
  leaves unused imports in __init__.py alone (possible re-exports).
- Strip shebangs from 72 importable modules. None of them are executable
  or invoked directly; the entry points are console_scripts and runpy.
- Flatten nested `with` blocks, collapse needless-bool returns, and apply
  the remaining mechanical ruff fixes (PIE810, FLY002, PERF102, FURB192,
  RUF059, I001).
- Pass check=False explicitly to the four subprocess.run() calls that
  inspect returncode themselves. That is the existing default.

Two rewrites are visible to mocks, so their tests move with them:
subprocess.run(stdout=PIPE, stderr=PIPE) became capture_output=True, and
open(path, "r", ...) lost the redundant mode.

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

tests/integration/test_recursive_capabilities.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 """
 Integration tests for recursive capability resolution and installer shadowing.
 
@@ -17,7 +15,6 @@ import shutil
 import tempfile
 import unittest
 from collections.abc import Sequence
-from typing import List, Tuple
 from unittest.mock import patch
 
 import pkgmgr.actions.install as install_mod
@@ -29,7 +26,7 @@ from pkgmgr.actions.install.installers.os_packages.arch_pkgbuild import (
 )
 from pkgmgr.actions.install.installers.python import PythonInstaller
 
-InstallerSpec = Tuple[str, object]
+InstallerSpec = tuple[str, object]
 
 
 class TestRecursiveCapabilitiesIntegration(unittest.TestCase):
@@ -54,7 +51,7 @@ class TestRecursiveCapabilitiesIntegration(unittest.TestCase):
         repo_dir: str,
         installers: Sequence[InstallerSpec],
         selected_repos=None,
-    ) -> List[str]:
+    ) -> list[str]:
         """
         Run install_repos() with a custom INSTALLERS list and capture which
         installer labels actually run.
@@ -69,7 +66,7 @@ class TestRecursiveCapabilitiesIntegration(unittest.TestCase):
         else:
             all_repos = selected_repos
 
-        called_installers: List[str] = []
+        called_installers: list[str] = []
 
         patched_installers = []
         for label, inst in installers: