perf(background-agent): use Set for countedToolPartIDs, cache circuit breaker settings, optimize loop detector

- Replace countedToolPartIDs string[] with Set<string> for O(1) has/add vs O(n) includes/spread (C2)
- Cache resolveCircuitBreakerSettings at manager level to avoid repeated object creation (C3)
- Optimize recordToolCall to avoid full array copy with slice (L1)

src/features/background-agent/loop-detector.ts

@@ -44,7 +44,9 @@ export function recordToolCall(
 ): ToolCallWindow {
   const previous = window?.toolSignatures ?? []
   const signature = createToolCallSignature(toolName, toolInput)
-  const toolSignatures = [...previous, signature].slice(-settings.windowSize)
+  const toolSignatures = previous.length >= settings.windowSize
+    ? [...previous.slice(1), signature]
+    : [...previous, signature]
 
   return {
     toolSignatures,