fix: resolve YAML syntax error in merge-bot workflow template literal

Fix template literal interpolation issue by extracting pr.base.ref to
a local variable before using it in the comment body. This prevents
YAML parsing errors with the GitHub Actions workflow.

The issue occurs when GitHub Actions tries to parse the workflow file
and encounters ${pr.base.ref} inside a JavaScript template literal,
which can be confused with GitHub Actions variable interpolation syntax.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: vilsonrodrigues

.github/workflows/merge-bot.yml

@@ -409,6 +409,7 @@ jobs:
               core.info('❌ PR is from fork - update not supported');
               core.setOutput('is_fork', 'true');
 
+              const baseBranch = pr.base.ref;
               await github.rest.issues.createComment({
                 owner: context.repo.owner,
                 repo: context.repo.repo,
@@ -418,8 +419,8 @@ jobs:
 To update your branch, please run these commands locally:
 
 \`\`\`bash
-git fetch upstream ${pr.base.ref}
-git merge upstream/${pr.base.ref}
+git fetch upstream ${baseBranch}
+git merge upstream/${baseBranch}
 git push
 \`\`\`