dsh-delivery-ledger: Scheduled Task Executor and Delivery Ledger

`dsh-delivery-ledger` is a plugin for DeepSeek Harness (DSH), designed to address scheduled task scheduling and delivery ledger requirements in automated agents. It employs window alignment and same-window deduplication to prevent duplicate executions caused by process jitter, and provides failure retry (resend) functionality. The plugin offers three core interfaces—`ledger_create`, `ledger_list`, and `ledger_retry`—and supports querying ledgers by status. Data is stored in local JSONL format with a zero-dependency design. In the current version, task scheduling exists only in memory, so registered tasks are lost after a process restart.