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_release_commands.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 """
 End-to-end style integration tests for the `pkgmgr release` CLI command.
 
@@ -149,9 +147,12 @@ class TestIntegrationReleaseCommand(unittest.TestCase):
         try:
             sys.argv = ["pkgmgr", "release", "--help"]
             # argparse will call sys.exit(), so we expect a SystemExit here.
-            with contextlib.redirect_stdout(buf), contextlib.redirect_stderr(buf):
-                with self.assertRaises(SystemExit) as cm:
-                    runpy.run_module("pkgmgr", run_name="__main__")
+            with (
+                contextlib.redirect_stdout(buf),
+                contextlib.redirect_stderr(buf),
+                self.assertRaises(SystemExit) as cm,
+            ):
+                runpy.run_module("pkgmgr", run_name="__main__")
         finally:
             sys.argv = original_argv