[FEATURE]: Plugins have no way to transform assistant text for display (or on completion) on v2 #53019

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Describe the enhancement you want to request

Use case

Some plugins have the model write small markers into its own replies. For example, a context-management plugin asks the model to tag messages with §N§ so it can refer back to them. These markers are meant for the plugin, not the user. On v1, experimental.text.complete lets a plugin rewrite an assistant text part when it completes, before it is saved, so the markers never reach the stored transcript or the UI.

On v2 there is no equivalent, so the markers are saved in session_message and shown in the timeline.

What v2 offers today (checked at upstream/v2 d1f8f5b, same as v2.0.22)

• SessionHooks (packages/plugin/src/promise/session.ts) is prompt, context, compaction, generate, title, model.request, http.request, http.response, experimental.ws.handshake, experimental.ws.send, experimental.ws.receive and retry. Nothing runs between the parsed model reply and persistence, or before rendering.
• The TUI plugin API's only markdown seam is markdown.registerCodeBlockRenderer(language, render) (packages/plugin/src/tui/context.ts), which handles fenced code blocks by language, not prose. The slot map has no per-message or per-part slot.
• The closest workaround is to replace the provider's raw response in http.response (or edit frames in experimental.ws.receive) and strip the markers from the wire stream before OpenCode parses it. That works, but it is provider-specific: a marker can be split across SSE events or JSON-escaped deltas, and every protocol (Anthropic, OpenAI Responses, Gemini, ...) needs its own parser.

Proposal (either would cover it; the first is preferred)

1. A display hook that runs before assistant text is rendered, in the TUI and the app, letting a plugin hide its own markers from what the user sees. The stored text and what is sent back to the model stay exactly as the provider returned them. This is the better fit because it doesn't change history: prompt-cache prefixes stay byte-stable, and the WebSocket continuation path, which compares stored output byte for byte, is unaffected.
2. A text.complete equivalent: a session hook that runs when an assistant text part completes, with the part text mutable, before it is persisted. This is what v1 does today.

Related issues

These cover nearby ground on v1 and are closed, so none of them tracks v2:

• Add TUI display-only chat text transform hook (string→string) #11864 (TUI display-only text transform hook): closed automatically after 90 days without activity, not implemented.
• [FEATURE] Plugin hook for streaming text transformation (e.g. text.delta) #39526 (streaming text.delta transform hook): closed as not planned.
• experimental.text.complete hook output not reflected in TUI display #7287: a v1 bug where experimental.text.complete output did not reach the TUI; fixed in v1, and that hook has no v2 equivalent.