feat: add base_branch input to specify source branch for new Claude branches (#72)

* feat: add base_branch input to specify source branch for new Claude branches

- Add base_branch input parameter to action.yml allowing users to specify which branch to use as source
- Update setupBranch function to accept and use the base branch parameter  
- Defaults to repository default branch if no base branch is specified
- Addresses issue #62 for better branch control

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* perf: optimize setupBranch to avoid unnecessary default branch fetch

Only fetch repository default branch when actually needed:
- Skip initial fetch when baseBranch is provided
- Fetch default branch at end only for return value and GitHub Actions output
- Eliminates unnecessary API call when users specify base branch

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* 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>

* refactor: move BASE_BRANCH env reading into parseGitHubContext

- Move BASE_BRANCH environment variable reading into parseGitHubContext for consistency
- Update setupBranch to use context.inputs.baseBranch instead of process.env
- Fix test descriptions to correctly reference BASE_BRANCH instead of DEFAULT_BRANCH
- Update test environment setup to use BASE_BRANCH

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

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

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>
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