fix: add packages section to release-please config (#39)

In manifest mode (config-file + manifest-file), release-please
requires an explicit packages section to know which paths to
release. Without it, Set(0) commits are found and no release
PR is created. Moving release-type and version-file under
packages['.'] fixes this.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: locus313

release-please-config.json

@@ -1,7 +1,11 @@
 {
   "$schema": "https://raw.githubusercontent.com/googleapis/release-please/main/schemas/config.json",
-  "release-type": "simple",
-  "version-file": "VERSION",
+  "packages": {
+    ".": {
+      "release-type": "simple",
+      "version-file": "VERSION"
+    }
+  },
   "include-v-in-tag": true,
   "changelog-sections": [
     {"type": "feat",     "section": "Features"},