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