Link the agent half of user/ on the Mac mini

Manifest lines can now name an OS. The SSH config, pwswitch and espanso are
Linux only, so link-user on the Mac links CLAUDE.md, rules, skills and the
opencode commands and nothing else. link-mcp no longer aborts under the Mac's
bash 3.2 when a server has no token.

setup/scripts/link-user

@@ -69,10 +69,12 @@ displace() {
 
 linked=0
 copied=0
+os="$(uname -s | tr '[:upper:]' '[:lower:]')"
 
-while read -r kind source destination; do
+while read -r kind source destination only_on _rest; do
     [[ -n "${kind:-}" ]] || continue
     [[ "$kind" == \#* ]] && continue
+    [[ -z "${only_on:-}" || "$only_on" == "$os" ]] || continue
 
     src="$USER_DIR/$source"
     dst="${destination/#\~/$HOME}"