fix: add support for pull_request_target event in GitHub Actions workflows (#579)

Add pull_request_target event support to enable Claude Code usage with forked
repositories while maintaining proper security boundaries. This resolves issues
with dependabot PRs and external contributions that require write permissions.

Changes:
- Add pull_request_target to supported GitHub events in context parsing
- Update type definitions to include PullRequestTargetEvent
- Modify IS_PR calculation to detect pull_request_target as PR context
- Add comprehensive test coverage for pull_request_target workflows
- Update documentation to reflect pull_request_target support

The pull_request_target event provides the same payload structure as
pull_request but runs with write permissions from the base repository,
making it ideal for secure automation of external contributions.

Fixes #347

src/create-prompt/index.ts

@@ -384,6 +384,7 @@ export function getEventTypeAndContext(envVars: PreparedContext): {
       };
 
     case "pull_request":
+    case "pull_request_target":
       return {
         eventType: "PULL_REQUEST",
         triggerContext: eventData.eventAction

src/create-prompt/types.ts

@@ -78,8 +78,7 @@ type IssueLabeledEvent = {
   labelTrigger: string;
 };
 
-type PullRequestEvent = {
-  eventName: "pull_request";
+type PullRequestBaseEvent = {
   eventAction?: string; // opened, synchronize, etc.
   isPR: true;
   prNumber: string;
@@ -87,6 +86,14 @@ type PullRequestEvent = {
   baseBranch?: string;
 };
 
+type PullRequestEvent = PullRequestBaseEvent & {
+  eventName: "pull_request";
+};
+
+type PullRequestTargetEvent = PullRequestBaseEvent & {
+  eventName: "pull_request_target";
+};
+
 // Union type for all possible event types
 export type EventData =
   | PullRequestReviewCommentEvent
@@ -96,7 +103,8 @@ export type EventData =
   | IssueOpenedEvent
   | IssueAssignedEvent
   | IssueLabeledEvent
-  | PullRequestEvent;
+  | PullRequestEvent
+  | PullRequestTargetEvent;
 
 // Combined type with separate eventData field
 export type PreparedContext = CommonFields & {

src/github/context.ts

@@ -174,7 +174,8 @@ export function parseGitHubContext(): GitHubContext {
         isPR: Boolean(payload.issue.pull_request),
       };
     }
-    case "pull_request": {
+    case "pull_request":
+    case "pull_request_target": {
       const payload = context.payload as PullRequestEvent;
       return {
         ...commonFields,