install.sh: add existing OEMDRV reuse, git origin conflict handling, refined permissions

- Detect existing OEMDRV partition at startup; offer to reuse it
  instead of creating a new one (mounts if needed, sources existing
  setup_system.inc.sh before cloning)
- When existing repo origin/branch differs from REPO_URL/REPO_BRANCH,
  offer to pull from existing origin, migrate to new origin (preserving
  gitignored local files), or fall through to fresh clone
- Extract finish_install() and do_clone_and_done() helpers to share
  clone, permissions, and configure.sh prompt across all paths
- Replace generic chmod with chown root:root + chmod ug=rwX,o=rX
  recursively, plus o+w on config/ and config.d/

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

system_setup/skel/pack_skel.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env sh
+# Usage: will make a tar-file from folder skel found in the directory where executed
+# If you want to change skel- content, extrakt your skel.tar.zstd to this directory, edit the files and use this script to repack
+source $(dirname "$0")/setup_system.inc.sh
+mv skel.tar.zst backup_skel.tar.zst
+if [ $? -eq 0 ]; then
+    echo "Old Archive renamed to backup_skel.tar.zst"
+else
+    echo "Some Errors occured, cannot continue."
+    exit 1
+fi
+tar -I 'zstd -9' -cf skel.tar.zst skel
+if [ $? -eq 0 ]; then
+    echo "Archive skel.tar.zst has been created"
+    echo "You should now remove skel- Folder here"
+else
+    echo "Some Errors occured, quit"
+    exit 1
+fi
+rm backup_skel.tar.zst
+echo "Old Archive deleted"
+# TODO
+# - up file to NC - is only possible, when setup already has the webdav-token created
+#