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/tools/background-task/create-background-output.blocking.test.ts

@@ -22,9 +22,9 @@ const mockContext = {
 function createTask(overrides: Partial<BackgroundTask> = {}): BackgroundTask {
   return {
     id: "task-1",
-    sessionID: "ses-1",
-    parentSessionID: "main-1",
-    parentMessageID: "msg-1",
+    sessionId: "ses-1",
+    parentSessionId: "main-1",
+    parentMessageId: "msg-1",
     description: "background task",
     prompt: "do work",
     agent: "test-agent",