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,

src/features/background-agent/manager-circuit-breaker.test.ts

@@ -233,7 +233,7 @@ describe("BackgroundManager circuit breaker", () => {
 
       expect(task.status).toBe("running")
       expect(task.progress?.toolCalls).toBe(1)
-      expect(task.progress?.countedToolPartIDs).toEqual(["tool-1"])
+      expect(task.progress?.countedToolPartIDs).toEqual(new Set(["tool-1"]))
     })
   })
 

src/features/background-agent/manager.ts

@@ -57,6 +57,7 @@ import {
   detectRepetitiveToolUse,
   recordToolCall,
   resolveCircuitBreakerSettings,
+  type CircuitBreakerSettings,
 } from "./loop-detector"
 import {
   createSubagentDepthLimitError,
@@ -152,6 +153,7 @@ export class BackgroundManager {
   private preStartDescendantReservations: Set<string>
   private enableParentSessionNotifications: boolean
   readonly taskHistory = new TaskHistory()
+  private cachedCircuitBreakerSettings?: CircuitBreakerSettings
 
   constructor(
     ctx: PluginInput,
@@ -918,7 +920,7 @@ export class BackgroundManager {
 
         task.progress.toolCalls += 1
         task.progress.lastTool = partInfo.tool
-        const circuitBreaker = resolveCircuitBreakerSettings(this.config)
+        const circuitBreaker = this.cachedCircuitBreakerSettings ?? (this.cachedCircuitBreakerSettings = resolveCircuitBreakerSettings(this.config))
         if (partInfo.tool) {
          task.progress.toolCallWindow = recordToolCall(
              task.progress.toolCallWindow,

src/features/background-agent/types.ts

@@ -19,7 +19,7 @@ export interface TaskProgress {
   toolCalls: number
   lastTool?: string
   toolCallWindow?: ToolCallWindow
-  countedToolPartIDs?: string[]
+  countedToolPartIDs?: Set<string>
   lastUpdate: Date
   lastMessage?: string
   lastMessageAt?: Date