dsh-mnemon-gc: Mnemon Memory GC Governance Plugin

The DSH plugin `dsh-mnemon-gc` is designed to address the growth of agent memory stores by integrating a conflict-driven correction mechanism and providing tiered memory governance with automatic inspection capabilities. The plugin classifies memory into three levels: `immune`, `superseded` (explicitly confirmed for deletion), and `watch` (under observation). By default, entries are marked as `superseded`, and soft deletion is performed only after confirmation. It supports configuration via CLI, tool invocation, and Web-based visualization, and provides read-only inspections and batch cleanup. The plugin depends on `dsh-mnemon >=0.2.9`, ensuring that deletion actions are well grounded and suitable for scenarios requiring fine-grained control over memory lifecycles.