feat(background-task): add 4 background task tools

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src/tools/background-task/constants.ts

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+export const BACKGROUND_TASK_DESCRIPTION = `Launch a background agent task that runs asynchronously.
+
+The task runs in a separate session while you continue with other work. The system will notify you when the task completes.
+
+Use this for:
+- Long-running research tasks
+- Complex analysis that doesn't need immediate results
+- Parallel workloads to maximize throughput
+
+Arguments:
+- description: Short task description (shown in status)
+- prompt: Full detailed prompt for the agent
+- agent: Agent type to use (any agent allowed)
+- session_id: Optional parent session ID (auto-detected if omitted)
+
+Returns immediately with task ID and session info. Use \`background_status\` to check progress.`
+
+export const BACKGROUND_STATUS_DESCRIPTION = `Check the status of background tasks.
+
+If taskId is provided, returns status for that specific task.
+If taskId is omitted, returns status for all tasks in the current session.
+
+Status includes:
+- Task description
+- Current status (pending/running/completed/error/cancelled)
+- Duration
+- Number of tool calls made
+- Last tool used
+
+Arguments:
+- taskId: Optional task ID. If omitted, lists all tasks for current session.`
+
+export const BACKGROUND_RESULT_DESCRIPTION = `Retrieve the result of a completed background task.
+
+Only works for tasks with status "completed". For running tasks, use \`background_status\` to check progress.
+
+Returns the full assistant output from the background session, including:
+- Task description
+- Duration
+- Complete response content
+- Session ID for reference
+
+Arguments:
+- taskId: Required task ID to retrieve result for.`
+
+export const BACKGROUND_CANCEL_DESCRIPTION = `Cancel a running background task.
+
+Only works for tasks with status "running". Aborts the background session and marks the task as cancelled.
+
+Arguments:
+- taskId: Required task ID to cancel.`