feat(install): offer /tmp on disk + SSH memory guard, deploy cloudpex helper

/tmp is a RAM-backed tmpfs on Ubuntu (50% of RAM). Agent runs fill it: half
the RAM goes, then every temp-file creation fails with ENOSPC and shells
break. Swap does not lift the cap, so the fix is /tmp on disk.

End-of-install offers (Linux, [y/N], skipped without a terminal, idempotent):
- offer_tmp_on_disk: mask tmp.mount + etc/tmpfiles.d/tmp.conf (wipe at
  boot, 10-day purge, /var/tmp rule kept). Effective at next reboot.
- offer_ssh_memory_guard: the previous server's rules. ssh.service drop-in
  (OOMScoreAdjust=-1000, MemoryMin=256M) + earlyoom with --avoid sshd and
  --prefer node/java. MemoryMin covers sshd only; earlyoom is the real guard.

install_cloudpex deploys the NAS mount helper in the Linux block.
Docs: README steps 12-14 + table, CLAUDE.md layout + lint command.

etc/systemd/ssh.service.d/override.conf

@@ -0,0 +1,8 @@
+# ssh.service drop-in: keep sshd alive when RAM runs out (rules of the previous server).
+# OOMScoreAdjust=-1000  the kernel OOM killer never selects sshd.
+# MemoryMin=256M        reclaim protection for the daemon's own cgroup. Login sessions
+#                       live in user.slice (logind), so this cannot reserve RAM for an
+#                       interactive shell — earlyoom is what frees memory in time.
+[Service]
+MemoryMin=256M
+OOMScoreAdjust=-1000