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/create-prompt/types.ts

@@ -16,7 +16,7 @@ type PullRequestReviewCommentEvent = {
   commentId?: string; // May be present for review comments
   commentBody: string;
   claudeBranch?: string;
-  defaultBranch?: string;
+  baseBranch?: string;
 };
 
 type PullRequestReviewEvent = {
@@ -25,7 +25,7 @@ type PullRequestReviewEvent = {
   prNumber: string;
   commentBody: string;
   claudeBranch?: string;
-  defaultBranch?: string;
+  baseBranch?: string;
 };
 
 type IssueCommentEvent = {
@@ -33,7 +33,7 @@ type IssueCommentEvent = {
   commentId: string;
   issueNumber: string;
   isPR: false;
-  defaultBranch: string;
+  baseBranch: string;
   claudeBranch: string;
   commentBody: string;
 };
@@ -46,7 +46,7 @@ type PullRequestCommentEvent = {
   isPR: true;
   commentBody: string;
   claudeBranch?: string;
-  defaultBranch?: string;
+  baseBranch?: string;
 };
 
 type IssueOpenedEvent = {
@@ -54,7 +54,7 @@ type IssueOpenedEvent = {
   eventAction: "opened";
   isPR: false;
   issueNumber: string;
-  defaultBranch: string;
+  baseBranch: string;
   claudeBranch: string;
 };
 
@@ -63,7 +63,7 @@ type IssueAssignedEvent = {
   eventAction: "assigned";
   isPR: false;
   issueNumber: string;
-  defaultBranch: string;
+  baseBranch: string;
   claudeBranch: string;
   assigneeTrigger: string;
 };
@@ -74,7 +74,7 @@ type PullRequestEvent = {
   isPR: true;
   prNumber: string;
   claudeBranch?: string;
-  defaultBranch?: string;
+  baseBranch?: string;
 };
 
 // Union type for all possible event types