Fix hooks to fail gracefully instead of blocking user prompts

Three issues fixed:

1. session-init handler: Worker 500 errors caused `throw`, which
   hook-command.ts caught and exited with code 2 (blocking error).
   This blocked the user's prompt entirely with:
   "Hook error: Error: Session initialization failed: 500"
   Fix: Log the failure and return SUCCESS so the prompt proceeds.

2. user-message handler: Referenced nonexistent constant
   HOOK_EXIT_CODES.USER_MESSAGE_ONLY (undefined). Also used
   console.error() for informational output, which Claude Code
   may flag as an error. Fix: Use process.stderr.write() and
   return HOOK_EXIT_CODES.SUCCESS explicitly.

3. Both handlers threw on HTTP errors from the worker, causing
   exit code 2 (blocking). Memory plugin failures should never
   prevent users from using Claude Code. All worker HTTP failures
   now log and return gracefully.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/handlers/session-init.ts

@@ -43,7 +43,9 @@ export const sessionInitHandler: EventHandler = {
     });
 
     if (!initResponse.ok) {
-      throw new Error(`Session initialization failed: ${initResponse.status}`);
+      // Log but don't throw - a worker 500 should not block the user's prompt
+      logger.failure('HOOK', `Session initialization failed: ${initResponse.status}`, { contentSessionId: sessionId, project });
+      return { continue: true, suppressOutput: true, exitCode: HOOK_EXIT_CODES.SUCCESS };
     }
 
     const initResult = await initResponse.json() as {
@@ -86,7 +88,8 @@ export const sessionInitHandler: EventHandler = {
       });
 
       if (!response.ok) {
-        throw new Error(`SDK agent start failed: ${response.status}`);
+        // Log but don't throw - SDK agent failure should not block the user's prompt
+        logger.failure('HOOK', `SDK agent start failed: ${response.status}`, { sessionDbId, promptNumber });
       }
     } else if (input.platform === 'cursor') {
       logger.debug('HOOK', 'session-init: Skipping SDK agent init for Cursor platform', { sessionDbId, promptNumber });