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.metadata.test.ts

@@ -20,9 +20,9 @@ describe("createBackgroundOutput metadata", () => {
 
     const task: BackgroundTask = {
       id: "task-1",
-      sessionID: undefined,
-      parentSessionID: "main-1",
-      parentMessageID: "msg-1",
+      sessionId: undefined,
+      parentSessionId: "main-1",
+      parentMessageId: "msg-1",
       description: "background task",
       prompt: "do work",
       agent: "test-agent",