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.

webui/static/htmx.min.js

@@ -0,0 +1 @@
+../../vendor/htmx-2.0.4.min.js

webui/static/json-enc.js

@@ -1,12 +0,0 @@
-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))
-  }
-})

webui/static/json-enc.js

@@ -0,0 +1 @@
+../../vendor/json-enc-2.0.0.js