refactor: colocate user interaction with ui packages

packages/ui/user-interaction/README.md

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+# @deepseek-ai/dsh-user-interaction
+
+Abstract user-interaction seam. It owns `ctx.userInteraction`, the service a model-facing tool or permission plugin uses when it needs to pause work and ask the human for a decision.
+
+## Service: `UserInteractionService` (ctx key: `userInteraction`)
+
+### Public API
+
+- `ctx.userInteraction.registerProvider(provider): () => void` Register the UI-side provider. Only one provider may be active in a context; disposal unregisters it.
+- `ctx.userInteraction.ask(request): Promise<AskUserQuestionAnswer>` Ask the active provider and wait for the answer.
+
+### Key Types
+
+- `AskUserQuestionRequest` — `{ questions: [{ id, question, header?, options?, multiSelect? }], agent?, signal? }`.
+- `AskUserQuestionOption` — `{ label, description? }`.
+- `AskUserQuestionAnswer` — `{ answers: [{ id, selected, custom? }] }`.
+- `UserInteractionProvider` — UI implementation with `ask(request)`.
+- `UserInteractionError` — `HarnessError` subclass with codes such as `EMPTY_QUESTIONS`, `NO_PROVIDER`, `DUPLICATE_PROVIDER`, and `ASK_ABORTED`.
+
+When an answer includes `custom`, `selected` is empty; custom text is an override rather than a supplement to selected choices.
+
+## Role
+
+This is the interface package. Model-facing consumers such as `@deepseek-ai/dsh-tool-ask-user` depend on this seam; UI front doors such as the `stdio-agent` readline module and the `acp` bridge provide the provider. The loop stays unchanged: a tool call simply awaits a promise, and the tool result resumes the normal agent loop.