fix: move signoff to top-level in release-please config (#47)

The signoff field is only read at the top level of release-please-config.json.
Placing it inside packages["."] was silently ignored, so the generated
chore(main): release X.Y.Z commits had no Signed-off-by trailer.

Signed-off-by: Logan Nguyen <lg.131.dev@gmail.com>

Author: quiet-node

release-please-config.json

@@ -1,13 +1,13 @@
 {
   "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
   "release-type": "node",
+  "signoff": "Logan Nguyen <lg.131.dev@gmail.com>",
   "packages": {
     ".": {
       "release-type": "node",
       "changelog-path": "CHANGELOG.md",
       "include-component-in-tag": false,
       "bump-minor-pre-major": true,
-      "signoff": "Logan Nguyen <lg.131.dev@gmail.com>",
       "extra-files": [
         {
           "type": "toml",