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/oracle-gap-closure.test.ts

@@ -131,7 +131,7 @@ describe("delegate-task Oracle gap closure", () => {
           description: "existing",
           agent: "explore",
           status: "running",
-          sessionID: "ses_bg_category",
+          sessionId: "ses_bg_category",
           category: "deep",
           model: MODEL,
         }),
@@ -163,7 +163,7 @@ describe("delegate-task Oracle gap closure", () => {
           description: "old desc",
           agent: "explore",
           status: "running",
-          sessionID: "ses_bg_title",
+          sessionId: "ses_bg_title",
           model: MODEL,
         }),
       },
@@ -221,7 +221,7 @@ describe("delegate-task Oracle gap closure", () => {
             description: "existing",
             agent: "explore",
             status: "running",
-            sessionID: "ses_bg_skills",
+            sessionId: "ses_bg_skills",
             model: MODEL,
           }
         },