refactor(omo-task): rename to call_omo_agent with run_in_background parameter

- Rename omo-task to call-omo-agent with mandatory run_in_background parameter
- Implement background mode using BackgroundManager (fire-and-forget abort)
- Implement sync mode with existing subagent logic
- Fix background_cancel: use fire-and-forget abort to prevent parent session interruption
- Add call_omo_agent to tool disable list in explore/librarian agents
- Add call_omo_agent to tool disable list in BackgroundManager

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/tools/background-task/tools.ts

@@ -196,13 +196,11 @@ export function createBackgroundCancel(manager: BackgroundManager, client: Openc
 Only running tasks can be cancelled.`
         }
 
-        const abortResult = await client.session.abort({
+        // Fire-and-forget: abort 요청을 보내고 await 하지 않음
+        // await 하면 메인 세션까지 abort 되는 문제 발생
+        client.session.abort({
           path: { id: task.sessionID },
-        })
-
-        if (abortResult.error) {
-          return `❌ Failed to abort session: ${(abortResult.error as any).message || String(abortResult.error)}`
-        }
+        }).catch(() => {})
 
         task.status = "cancelled"
         task.completedAt = new Date()