fix: gracefully handle empty prompts in session-init hook

When a user submits an empty or whitespace-only prompt to Claude Code,
the UserPromptSubmit hook would throw "sessionInitHandler requires prompt"
and block the operation.

This change returns early with `{ continue: true }` instead of throwing,
allowing the session to continue gracefully.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/cli/handlers/session-init.ts

@@ -21,8 +21,9 @@ export const sessionInitHandler: EventHandler = {
 
     const { sessionId, cwd, prompt } = input;
 
-    if (!prompt) {
+    // Gracefully handle empty prompts (user submitted blank input)
-      throw new Error('sessionInitHandler requires prompt');
+    if (!prompt || !prompt.trim()) {
+      return { continue: true, suppressOutput: true };
     }
 
     const project = getProjectName(cwd);