[EPIC] Workspace Activity Rules

### Certification

- [x] I certify I am an Epic Owner for Kubeflow Notebooks 2.0 and expected to create planning-related issues.

### User Story

As an ML Platform Admin, Joel needs to efficiently manage and optimize the resources available on the platform. He requires a mechanism to automatically identify and terminate idle notebook pods to prevent resource waste, especially expensive resources like GPUs. This "culling" feature allows him to maintain a cost-effective and performant environment for all users by reclaiming underutilized resources, ensuring that they are available for active workloads. By implementing Activity Rules, Joel can proactively address potential resource constraints and ensure the smooth operation of the platform, ultimately providing a better experience for AI/ML practitioners like Bella.

This epic only describes the desired Activity Rules behavior for Kubeflow notebook pods; it names no files, tests, or entry points. Start by locating the notebook pod lifecycle and resource-management components, then clarify the idle criteria, termination behavior, and validation needed for automatic culling to be considered complete.

Written by the indexing model from the issue text.