refactor: move JS vendors to vendor/ with versioned filenames

Vendor JS files to vendor/ with versioned names, symlinks in
webui/static/ select active version. Update update-vendor.sh and
AGENTS.md.

vendor/json-enc-2.0.0.js

@@ -0,0 +1,12 @@
+htmx.defineExtension('json-enc', {
+  onEvent: function(name, evt) {
+    if (name === 'htmx:configRequest') {
+      evt.detail.headers['Content-Type'] = 'application/json'
+    }
+  },
+
+  encodeParameters: function(xhr, parameters, elt) {
+    xhr.overrideMimeType('text/json')
+    return (JSON.stringify(parameters))
+  }
+})