dsh-agent-policy: DeepSeek Harness Policy Contract and MCP Initialization Defaults

dsh-agent-policy is a shared policy plugin for DeepSeek Harness (DSH), designed to address issues such as fragmented policy definitions, version inconsistency, and difficulty reusing configurations in agent development. It does not intercept tools; instead, it provides schemas, preset base classes, and MCP default behaviors for other plugins to consume. The plugin supports configuring preset templates (such as research) and MCP panel default values in `defaults.json`. Permission control is handled by `dsh-agent-gate`; uninstalling the plugin does not relax permissions, and configuration data remains persisted.