feat(install): add --update to re-run active-layer installers and improve Nix refresh logic

* Add `force_update` to `RepoContext` and propagate it through install/update flows
* Add `pkgmgr install --update` to force re-running installers even if the same CLI layer is already loaded
* Enhance `NixFlakeInstaller` to ensure correct outputs (pkgmgr + optional default for package-manager) and support refresh/upgrade with index-based fallback remove+reinstall
* Make Python/Makefile installers emit an “upgraded” marker when `force_update` is used
* Add E2E tests for “three times install” scenarios (makefile, nix, venv) with shared run helper
* Fix git safe.directory wildcard quoting in E2E shell runner and minor cleanup/reordering of imports/comments

https://chatgpt.com/share/693db0b4-6ea4-800f-b44a-f03939c7fb9e

tests/e2e/_util.py

@@ -0,0 +1,24 @@
+import subprocess
+
+
+def run(cmd, *, cwd=None, env=None, shell=False) -> str:
+    proc = subprocess.run(
+        cmd,
+        cwd=cwd,
+        env=env,
+        shell=shell,
+        text=True,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+    )
+
+    print("----- BEGIN COMMAND -----")
+    print(cmd if isinstance(cmd, str) else " ".join(cmd))
+    print("----- OUTPUT -----")
+    print(proc.stdout.rstrip())
+    print("----- END COMMAND -----")
+
+    if proc.returncode != 0:
+        raise AssertionError(proc.stdout)
+
+    return proc.stdout