[Bug]: V2 removed UI-only session messages available through ignored + noReply

Description

V1 plugins could display a session message without starting the agent and without adding the text to model context:

await client.session.prompt({
  path: { id: sessionID },
  body: {
    noReply: true,
    parts: [{ type: 'text', text: notice, ignored: true }],
  },
})

This was useful for plugin status messages such as provider failover and cooldown notices.

V2 removed noReply and ignored text parts. The closest API is:

await client.session.synthetic({
  sessionID,
  text: notice,
  description: 'Provider fallback',
  resume: false,
})

resume: false prevents an immediate execution, but the synthetic input is later promoted into the transcript as a user-role model message. Compaction also retains it as synthetic context.

V2 needs a way to add a visible session notice that:

• does not start or resume execution
• is rendered by TUI, Desktop, and Web clients
• never enters model context or compaction

This is narrower than #6330. It restores behavior available to V1 server plugins.

Plugins

@subrouter/opencode

OpenCode version

V1: 1.18.23

V2: 2.0.2

Steps to reproduce

1. Call session.synthetic() with description and resume: false.
2. Start the next session execution.
3. Inspect the provider request.
4. The synthetic text appears as a user-role model message.
5. There is no V2 equivalent of the V1 ignored: true plus noReply: true behavior.

Screenshot and/or share link

N/A

Operating System

macOS

Terminal

N/A, plugin SDK