install.sh: respect /ext disabled state on re-run

When linking, check for <name>.ts.off pointing into this repo and skip
relinking if found. Means a previously /ext-disabled extension stays
disabled across install.sh re-runs (e.g. when adding a new extension).

README + AGENTS updated with the new behavior.

install.sh

@@ -151,9 +151,17 @@ do_install() {
         in_install_set "$bare" || continue
 
         local dest="${EXTENSIONS_DEST}/${name}"
+        local disabled="${dest}.off"
 
         note "Linking ${name}"
 
+        # Respect a prior /ext disable: if <name>.ts.off exists and points
+        # into this repo, leave it alone. ext-toggle will flip it back.
+        if [[ -L "$disabled" ]] && link_into_repo "$disabled"; then
+            ok "${name} kept disabled (${name}.off present)"
+            continue
+        fi
+
         if [[ -e "$dest" || -L "$dest" ]]; then
             if link_into_repo "$dest"; then
                 ok "${name} already linked"