Add changeset for OpenCode command reference fix (#627)

Author: TabishB

.changeset/opencode-command-references.md

@@ -0,0 +1,7 @@
+---
+"@fission-ai/openspec": patch
+---
+
+### Bug Fixes
+
+- **OpenCode command references** — Command references in generated files now use the correct `/opsx-` hyphen format instead of `/opsx:` colon format, ensuring commands work properly in OpenCode