Setup: create agent users with an empty skeleton (useradd -k), so agent homes start with no outward links (follow-up to #899) #922

Follow-up to #899 (#798), from #899's round-3 review: #899 (comment)

Problem. Setup creates agent users with useradd --create-home, which copies /etc/skel (scripts/setup/lib.sh:67). A host whose skeleton holds outward links (CI's runner has ~/.ghcup -> /usr/local/.ghcup, root-owned and world-writable) puts those links into every agent home. That is why #899 can't simply refuse every outward link: it has to pin the directories along such a link's path and bind its target read-only. It also leaves a measured cross-user residual: another user's sandboxed command can rewrite a world-writable target that an agent's home links to.

Wanted.

• Create agent users with an empty skeleton: useradd -m -k <empty dir>, an empty root-owned 0755 directory. The agent's home then starts with no outward links, and the strict rule (refuse any outward link through a writable hop) costs nothing on new installs.
• Existing installs: doctor reports outward links in agent homes, with the exact root fix.
• Tests that fail without it:
  • a new agent user's home contains no symlinks;
  • doctor flags a planted outward link.

Protected: setup scripts are in the critical set.