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/delegate-task/background-continuation.test.ts

@@ -9,7 +9,7 @@ describe("executeBackgroundContinuation - subagent metadata", () => {
         description: "oracle consultation",
         agent: "oracle",
         status: "running",
-        sessionID: "ses_resumed_123",
+        sessionId: "ses_resumed_123",
       }),
     }
 
@@ -55,7 +55,7 @@ describe("executeBackgroundContinuation - subagent metadata", () => {
         description: "unknown task",
         agent: undefined,
         status: "running",
-        sessionID: "ses_resumed_456",
+        sessionId: "ses_resumed_456",
       }),
     }