refactor(nix): split NixFlakeInstaller into atomic modules and add GitHub 403 retry handling

- Move Nix flake installer into installers/nix/ with atomic components
  (installer, runner, profile, retry, types)
- Preserve legacy behavior and semantics of NixFlakeInstaller
- Add GitHub API 403 rate-limit retry with Fibonacci backoff + jitter
- Update all imports to new nix module path
- Rename legacy unit tests and adapt patches to new structure
- Add unit test for simulated GitHub 403 retry without realtime sleeping

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

tests/integration/test_recursive_capabilities.py

@@ -23,7 +23,7 @@ from unittest.mock import patch
 import pkgmgr.actions.install as install_mod
 from pkgmgr.actions.install import install_repos
 from pkgmgr.actions.install.installers.makefile import MakefileInstaller
-from pkgmgr.actions.install.installers.nix_flake import NixFlakeInstaller
+from pkgmgr.actions.install.installers.nix import NixFlakeInstaller
 from pkgmgr.actions.install.installers.os_packages.arch_pkgbuild import (
     ArchPkgbuildInstaller,
 )