fix: use available model for release changelog (#51773)

Co-authored-by: rekram1-node <rekram1-node@users.noreply.github.com>

Author: opencode-agent[bot]

.opencode/command/changelog.md

@@ -1,5 +1,5 @@
 ---
-model: opencode/gpt-5.4
+model: opencode/gpt-6-luna
 ---
 
 Create `UPCOMING_CHANGELOG.md` from the structured changelog input below.