fix(nix): resolve Ruff F821 via TYPE_CHECKING and stabilize NixFlakeInstaller tests

- Add TYPE_CHECKING imports for RepoContext and CommandRunner to avoid runtime deps
- Fix Ruff F821 undefined-name errors in nix installer modules
- Refactor legacy NixFlakeInstaller unit tests to mock subprocess.run directly
- Remove obsolete run_cmd_mock usage and assert install calls via subprocess calls
- Ensure tests run without realtime waits or external nix dependencies

https://chatgpt.com/share/693e925d-a79c-800f-b0b6-92b8ba260b11

src/pkgmgr/actions/install/installers/nix/installer.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 
 import os
 import shutil
-from typing import List, Tuple
+from typing import List, Tuple, TYPE_CHECKING
 
 from pkgmgr.actions.install.installers.base import BaseInstaller
 
@@ -11,6 +11,8 @@ from .profile import NixProfileInspector
 from .retry import GitHubRateLimitRetry, RetryPolicy
 from .runner import CommandRunner
 
+if TYPE_CHECKING:
+    from pkgmgr.actions.install.context import RepoContext
 
 class NixFlakeInstaller(BaseInstaller):
     layer = "nix"

src/pkgmgr/actions/install/installers/nix/profile.py

@@ -1,9 +1,13 @@
-# src/pkgmgr/actions/install/installers/nix/profile.py
 from __future__ import annotations
 
 import json
-from typing import Any, List
+from typing import Any, List, TYPE_CHECKING
 
+
+if TYPE_CHECKING:
+    from pkgmgr.actions.install.context import RepoContext
+    from .runner import CommandRunner
+ 
 class NixProfileInspector:
     """
     Reads and interprets `nix profile list --json` and provides helpers for

src/pkgmgr/actions/install/installers/nix/retry.py

@@ -1,13 +1,15 @@
-# src/pkgmgr/actions/install/installers/nix/retry.py
 from __future__ import annotations
 
 import random
 import time
 from dataclasses import dataclass
-from typing import Iterable
+from typing import Iterable, TYPE_CHECKING
 
 from .types import RunResult
 
+if TYPE_CHECKING:
+    from pkgmgr.actions.install.context import RepoContext
+    from .runner import CommandRunner
 
 @dataclass(frozen=True)
 class RetryPolicy:

src/pkgmgr/actions/install/installers/nix/runner.py

@@ -2,8 +2,12 @@ from __future__ import annotations
 
 import subprocess
 
+from typing import TYPE_CHECKING
+
 from .types import RunResult
 
+if TYPE_CHECKING:
+    from pkgmgr.actions.install.context import RepoContext
 
 class CommandRunner:
     """