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/e2e/test_version_commands.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 """
 End-to-end tests for the `pkgmgr version` command.
 
@@ -25,7 +23,6 @@ import os
 import runpy
 import sys
 import unittest
-from typing import List
 
 from pkgmgr.core.config.load import load_config
 
@@ -54,7 +51,7 @@ def _load_pkgmgr_repo_dir() -> str:
     directories = cfg.get("directories", {})
     base_repos_dir = os.path.expanduser(directories.get("repositories", ""))
 
-    candidates: List[dict] = cfg.get("repositories", []) or []
+    candidates: list[dict] = cfg.get("repositories", []) or []
     for repo in candidates:
         repo_name = (repo.get("repository") or "").strip()
         alias = (repo.get("alias") or "").strip()