dsh-plan-preset: Map DSH Read-Only to Plan Mode

`dsh-plan-preset` is a lightweight configuration plugin for DeepSeek Harness (DSH). It leverages the Cordis Patch mechanism to map the DSH Web UI's "read-only" permission to an OpenCode-style "Plan" mode. Once installed, this mode enforces strict semantic isolation: file operations are restricted to read-only access, web search is available, and any privilege escalation requests are prohibited. This addresses the differing permission requirements between an agent's "research/planning" and "execution" phases, blocks out-of-scope operations, and provides teams with a standardized workflow permission management solution.