fix: add empty output guard for OpenCode reviewer

Addresses review feedback — checks if opencode output file is
non-empty after invocation, writes a failure message if empty
to prevent blank sections in REVIEWS.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

get-shit-done/workflows/review.md

@@ -147,6 +147,9 @@ coderabbit review --prompt-only 2>/dev/null > /tmp/gsd-review-coderabbit-{phase}
 **OpenCode (via GitHub Copilot):**
 ```bash
 cat /tmp/gsd-review-prompt-{phase}.md | opencode run - 2>/dev/null > /tmp/gsd-review-opencode-{phase}.md
+if [ ! -s /tmp/gsd-review-opencode-{phase}.md ]; then
+  echo "OpenCode review failed or returned empty output." > /tmp/gsd-review-opencode-{phase}.md
+fi
 ```
 
 If a CLI fails, log the error and continue with remaining CLIs.