fix: gracefully handle missing input fields in hook handlers (#1098)

The summarize (Stop) and observation (PostToolUse) handlers throw
blocking errors (exit code 2) when optional input fields like
transcriptPath, toolName, or cwd are missing. This causes visible
hook errors on every session stop and after some tool uses.

Replace throws with graceful returns matching the existing pattern
used for worker-unavailable checks.

Fixes #1097

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/handlers/observation.ts

@@ -24,7 +24,8 @@ export const observationHandler: EventHandler = {
     const { sessionId, cwd, toolName, toolInput, toolResponse } = input;
 
     if (!toolName) {
-      throw new Error('observationHandler requires toolName');
+      // No tool name provided - skip observation gracefully
+      return { continue: true, suppressOutput: true, exitCode: HOOK_EXIT_CODES.SUCCESS };
     }
 
     const port = getWorkerPort();