DSH (DeepSeek Harness) Web GUI plugin

Key features

• Registers one client plugin (dsh.client manifest, platform: web) that contributes an entry to the official conversation.input.left composer tool-row slot (a documented multi-plugin list slot — zero official changes).
• The button renders only while the session is idle, a tail user/steering message exists, and that message is plain text (draft images are browser File-backed and cannot be resurrected from the durable log).
• On clickinputActions.setDraft(text) restores the text and the composer is focused.
• The plugin prefers inputActions.focus when the deployed runtime exposes it, otherwise falls back to the composer textarea (the GUI keeps exactly one).
• If you remove the plugin (dsh plugin remove dsh-plugin-edit-message), the surface disappears entirely — the tool row reverts to official chrome.