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/unit/pkgmgr/cli/test_handle_branch.py

@@ -45,7 +45,7 @@ class TestCliBranch(unittest.TestCase):
         handle_branch(args, ctx)
 
         mock_open_branch.assert_called_once()
-        call_args, call_kwargs = mock_open_branch.call_args
+        _call_args, call_kwargs = mock_open_branch.call_args
         self.assertEqual(call_kwargs.get("name"), "feature/cli-test")
         self.assertEqual(call_kwargs.get("base_branch"), "develop")
         self.assertEqual(call_kwargs.get("cwd"), ".")