refactor(background-agent): normalize task ID field naming

Rename BackgroundTask and attempt ID fields to camelCase across background-agent consumers while moving BackgroundManager construction to a single config object.

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

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

src/features/background-agent/default-message-staleness-timeout.test.ts

@@ -8,9 +8,9 @@ import type { BackgroundTask } from "./types"
 function createRunningTask(startedAt: Date): BackgroundTask {
   return {
     id: "task-1",
-    sessionID: "ses-1",
-    parentSessionID: "parent-ses-1",
-    parentMessageID: "msg-1",
+    sessionId: "ses-1",
+    parentSessionId: "parent-ses-1",
+    parentMessageId: "msg-1",
     description: "test",
     prompt: "test",
     agent: "explore",