fix: allow direct_prompt with issue assignment without requiring assignee_trigger (#192)

- Modified validation logic to only require assignee_trigger when direct_prompt is not provided
- Made assigneeTrigger optional in IssueAssignedEvent type definition
- Enhanced context generation to handle missing assigneeTrigger gracefully
- Added comprehensive test coverage for the new behavior

This enables direct_prompt workflows on issue assignment events without
requiring assignee_trigger configuration, fixing the error:
"ASSIGNEE_TRIGGER is required for issue assigned event"

Fixes #113

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

Co-authored-by: Claude <noreply@anthropic.com>

src/create-prompt/types.ts

@@ -65,7 +65,7 @@ type IssueAssignedEvent = {
   issueNumber: string;
   baseBranch: string;
   claudeBranch: string;
-  assigneeTrigger: string;
+  assigneeTrigger?: string;
 };
 
 type PullRequestEvent = {