dsh-subagent-parent-message: Add bidirectional message channel for delegated sub-agents

When delegating tasks using DSH, the core `send_message` only allows persistent and repliable child agents to contact their direct parent. However, the parent session is often shadowed by the team version of `send_message` (target schema), masking the same-named tool. Workflow one-time child agents lack activation continuation, causing the core channel to fail directly. The traditional approach involves writing results to disk for the parent to read. This plugin (maintained by zhyunran, MIT, requires DSH 0.2.x, currently 0.1.2) provides two namespace tools: `subagent_parent_message`, which allows child agents to parse the direct parent from the Session header and report via `agent.followup/steer + flush`, supporting both followup and steer modes; and `subagent_send_message`, a thin adapter of `ctx.subagents.sendMessage()` for parent sessions to contact direct repliable child agents. Neither tool waits for acknowledgments; the former does not rely on activation continuation, thereby supporting workflow one-time child agents. The article covers pain points, features, installation commands, typical usage, applicable scenarios, and limitations, while also noting that the plugin runs with the current DSH process permissions and advising users to check the source code and license before installation.