dsh-user-steer: A Mid-Run User Steering Plugin for DeepSeek Harness

`dsh-user-steer` is a user mid-course steering plugin for DeepSeek Harness (DSH). It addresses the challenge that directly appending messages during agent task execution can interrupt tool calls or break contextual coherence. The plugin provides multiple methods, including `/steer` (prominent interjection), `/inject` (silent injection), and injection bars. By queuing instructions and processing them at tool batch boundaries, it ensures guidance is safe and non-disruptive to the current process while enabling precise course correction.