chore: bump next to 1.5.1-dev.0

Move next onto the -dev prerelease stream after the v1.5.0 release per
ADR-660 (next must not rest at the last-released version).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "gsd-core",
   "displayName": "GSD Core",
-  "version": "1.5.0",
+  "version": "1.5.1-dev.0",
   "description": "GSD Core is a meta-prompting, context engineering, and spec-driven development system for AI coding agents.",
   "author": {
     "name": "open-gsd",