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/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"]))
     })
   })