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/entrypoints/update-comment-link.ts

@@ -18,7 +18,7 @@ async function run() {
     const commentId = parseInt(process.env.CLAUDE_COMMENT_ID!);
     const githubToken = process.env.GITHUB_TOKEN!;
     const claudeBranch = process.env.CLAUDE_BRANCH;
-    const defaultBranch = process.env.DEFAULT_BRANCH || "main";
+    const baseBranch = process.env.BASE_BRANCH || "main";
     const triggerUsername = process.env.TRIGGER_USERNAME;
 
     const context = parseGitHubContext();
@@ -92,7 +92,7 @@ async function run() {
       owner,
       repo,
       claudeBranch,
-      defaultBranch,
+      baseBranch,
     );
 
     // Check if we need to add PR URL when we have a new branch
@@ -102,7 +102,7 @@ async function run() {
       // Check if comment already contains a PR URL
       const serverUrlPattern = serverUrl.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
       const prUrlPattern = new RegExp(
-        `${serverUrlPattern}\\/.+\\/compare\\/${defaultBranch.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}\\.\\.\\.`,
+        `${serverUrlPattern}\\/.+\\/compare\\/${baseBranch.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}\\.\\.\\.`,
       );
       const containsPRUrl = currentBody.match(prUrlPattern);
 
@@ -113,7 +113,7 @@ async function run() {
             await octokit.rest.repos.compareCommitsWithBasehead({
               owner,
               repo,
-              basehead: `${defaultBranch}...${claudeBranch}`,
+              basehead: `${baseBranch}...${claudeBranch}`,
             });
 
           // If there are changes (commits or file changes), add the PR URL
@@ -128,7 +128,7 @@ async function run() {
             const prBody = encodeURIComponent(
               `This PR addresses ${entityType.toLowerCase()} #${context.entityNumber}\n\nGenerated with [Claude Code](https://claude.ai/code)`,
             );
-            const prUrl = `${serverUrl}/${owner}/${repo}/compare/${defaultBranch}...${claudeBranch}?quick_pull=1&title=${prTitle}&body=${prBody}`;
+            const prUrl = `${serverUrl}/${owner}/${repo}/compare/${baseBranch}...${claudeBranch}?quick_pull=1&title=${prTitle}&body=${prBody}`;
             prLink = `\n[Create a PR](${prUrl})`;
           }
         } catch (error) {