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/unstable-agent-permission.test.ts

@@ -11,7 +11,7 @@ describe("executeUnstableAgentTask session permission", () => {
         launchCalls.push(input)
         return {
           id: "bg_unstable_permission",
-          sessionID: "ses_unstable_permission",
+          sessionId: "ses_unstable_permission",
           description: "test task",
           agent: "sisyphus-junior",
           status: "running",
@@ -19,7 +19,7 @@ describe("executeUnstableAgentTask session permission", () => {
       },
       getTask: () => ({
         id: "bg_unstable_permission",
-        sessionID: "ses_unstable_permission",
+        sessionId: "ses_unstable_permission",
         status: "interrupt",
         description: "test task",
         agent: "sisyphus-junior",