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.

scripts/update-vendor.sh

@@ -1,6 +1,5 @@
 #!/usr/bin/env bash
-# Download and vendor frontend JS libraries into webui/static/
-# and backend binaries into vendor/.
+# Download and vendor libraries into vendor/.
 # Run from the project root after updating the VERSION variables below.
 set -euo pipefail
 
@@ -9,7 +8,8 @@ HTMX_VERSION="2.0.4"
 HTMX_JSON_ENC_VERSION="2.0.0"
 ACME_VERSION="3.1.3"
 
-STATIC_DIR="webui/static"
+VENDOR="vendor"
+STATIC="webui/static"
 
 download() {
     local name="$1" url="$2" dest="$3"
@@ -23,16 +23,20 @@ download() {
 
 download "htmx@${HTMX_VERSION}" \
     "https://unpkg.com/htmx.org@${HTMX_VERSION}/dist/htmx.min.js" \
-    "${STATIC_DIR}/htmx.min.js"
+    "${VENDOR}/htmx-${HTMX_VERSION}.min.js"
 
 download "htmx-ext-json-enc@${HTMX_JSON_ENC_VERSION}" \
     "https://unpkg.com/htmx-ext-json-enc@${HTMX_JSON_ENC_VERSION}/json-enc.js" \
-    "${STATIC_DIR}/json-enc.js"
+    "${VENDOR}/json-enc-${HTMX_JSON_ENC_VERSION}.js"
 
 download "acme.sh@${ACME_VERSION}" \
     "https://raw.githubusercontent.com/acmesh-official/acme.sh/${ACME_VERSION}/acme.sh" \
-    "vendor/acme.sh"
+    "${VENDOR}/acme.sh"
 
-chmod +x "vendor/acme.sh"
+chmod +x "${VENDOR}/acme.sh"
+
+# Symlinks in webui/static/ select the active version
+ln -sf "../../vendor/htmx-${HTMX_VERSION}.min.js" "${STATIC}/htmx.min.js"
+ln -sf "../../vendor/json-enc-${HTMX_JSON_ENC_VERSION}.js" "${STATIC}/json-enc.js"
 
 echo "[done] All libraries vendored."