chore(#660): bump next to 1.3.1-dev.0 (-dev stream per ADR 660) (#672)

After the 1.3.0 release, next moves onto the -dev prerelease stream so the
trunk self-identifies as unreleased (floor = next patch). First manual
exercise of the ADR-660 release model.

Refs #660

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@opengsd/gsd-core",
-  "version": "1.3.0",
+  "version": "1.3.1-dev.0",
   "description": "GSD Core is a meta-prompting, context engineering, and spec-driven development system for AI coding agents.",
   "bin": {
     "gsd-core": "bin/install.js",