fix: address medium severity issues from code review

- Fix inconsistent null handling in state-store (use ?? instead of ||)
- Add JSON.stringify error handling with specific messages
- Add error handling for SSE init event in app.js
- Fix spawn orchestrator silent failures (add logging)
- Fix statSync race condition in subagent-watcher
- Fix unbounded _taskNumberToContent Map in ralph-tracker
- Fix YAML pattern case sensitivity in ralph-config

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

src/subagent-watcher.ts

@@ -798,8 +798,14 @@ export class SubagentWatcher extends EventEmitter {
 
     const agentId = basename(filePath).replace('agent-', '').replace('.jsonl', '');
 
-    // Initial info
-    const stat = statSync(filePath);
+    // Initial info - handle race condition where file may be deleted between discovery and stat
+    let stat;
+    try {
+      stat = statSync(filePath);
+    } catch {
+      // File was deleted between discovery and stat - skip this agent
+      return;
+    }
 
     // Extract description - prefer reading from parent transcript (most reliable)
     // The parent transcript has the exact Task tool call with description parameter