steer_subagent: Insert a directive into the current turn of a background subagent

`steer_subagent` is a plugin for DeepSeek Harness (DSH) used to directly intervene while a background subagent is running. It supports two modes, “interject” and “interrupt,” allowing a turn to be force-restarted without rolling back completed steps. The plugin provides an installation command and allows adjusting the tool name by overriding configuration options. A typical use case is inserting instructions mid-run while a subagent is counting. However, it depends on the removed `ctx.subagents.followup` service and was marked deprecated on 2026-09-05; evaluating the official `send_message` capability is recommended in new environments.