refactor(background-agent): standardize loop detector null guards

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -36,7 +36,7 @@ export function recordToolCall(
   settings: CircuitBreakerSettings,
   toolInput?: Record<string, unknown> | null
 ): ToolCallWindow {
-  if (toolInput === undefined || toolInput === null) {
+  if (toolInput == null) {
     return {
       lastSignature: `${toolName}::__unknown-input__`,
       consecutiveCount: 1,
@@ -78,7 +78,7 @@ export function createToolCallSignature(
   toolName: string,
   toolInput?: Record<string, unknown> | null
 ): string {
-  if (toolInput === undefined || toolInput === null) {
+  if (toolInput == null) {
     return toolName
   }
   if (Object.keys(toolInput).length === 0) {