fix: properly handle base branch throughout the action workflow

- Fix TypeScript error where defaultBranch was used before being assigned
- Replace DEFAULT_BRANCH with BASE_BRANCH in subsequent workflow steps
- Update PR creation and branch comparison to use the actual base branch
- Ensure custom base_branch input is respected in all operations

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

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

src/github/operations/branch-cleanup.ts

@@ -6,7 +6,7 @@ export async function checkAndDeleteEmptyBranch(
   owner: string,
   repo: string,
   claudeBranch: string | undefined,
-  defaultBranch: string,
+  baseBranch: string,
 ): Promise<{ shouldDeleteBranch: boolean; branchLink: string }> {
   let branchLink = "";
   let shouldDeleteBranch = false;
@@ -18,7 +18,7 @@ export async function checkAndDeleteEmptyBranch(
         await octokit.rest.repos.compareCommitsWithBasehead({
           owner,
           repo,
-          basehead: `${defaultBranch}...${claudeBranch}`,
+          basehead: `${baseBranch}...${claudeBranch}`,
         });
 
       // If there are no commits, mark branch for deletion