*fix(nix): resolve nix profile conflicts without numeric indices and fix update pkgmgr system test*

* Switch conflict handling from index-based removal to token-based removal (*nix profile remove <name>*) for newer nix versions
* Add robust parsing of *nix profile list --json* with normalization and heuristics for output/name matching
* Detect at runtime whether numeric profile indices are supported and fall back automatically when they are not
* Ensure *pkgmgr* / *package-manager* flake outputs are correctly identified and cleaned up during reinstall
* Fix failing E2E test *test_update_pkgmgr_shallow_pkgmgr_with_system* by reliably removing conflicting profile entries before reinstall

https://chatgpt.com/share/693efae5-b8bc-800f-94e3-28c93b74ed7b

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

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import List, Optional
+
+
+@dataclass(frozen=True)
+class NixProfileEntry:
+    """
+    Minimal normalized representation of one nix profile element entry.
+    """
+
+    key: str
+    index: Optional[int]
+    name: str
+    attr_path: str
+    store_paths: List[str]