Add deixis UIWorker example

A ReplyToolMixin UIWorker that grounds in the user's text selection (the
<selection> block in the snapshot) and points back via select_text — both
directions of deictic reference.

examples/multi-worker/ui-worker/deixis/README.md

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+# deixis
+
+The UIWorker grounds in what the user just selected. Highlight a
+paragraph in the article and ask "explain this" — the worker reads your
+selection from the snapshot and answers about that specific content.
+
+## What it shows
+
+- The **read direction**: the client captures `window.getSelection()`
+  and emits a `<selection ref="...">selected text</selection>` block
+  inside `<ui_state>`. The `UIWorker` treats it as the deictic referent
+  for "this", "that", "this paragraph". Asking "what does this mean?"
+  with a paragraph selected resolves cleanly.
+- The **write direction**: the worker says "this paragraph" and issues a
+  `select_text=ref` command. The client puts the page's text selection
+  on that element, so the user sees exactly which paragraph the worker
+  means.
+- `ReplyToolMixin` covering reading-style apps: the same bundled tool
+  pointing uses also has `select_text` (durable selection) alongside
+  `highlight` (brief flash).
+
+## What it adds vs. `pointing`
+
+`pointing` proved the worker can act visually on the page (scroll,
+highlight). This one proves it can read the user's pointer (text
+selection) and point back in the same idiom (programmatic selection).
+Same skeleton; the new parts are the `select_text` command and the
+matching client handler.
+
+## Run
+
+Two terminals.
+
+**Terminal 1 — bot:**
+
+```bash
+cd examples/multi-worker/ui-worker/deixis
+uv run python bot.py
+```
+
+The bot starts on `http://localhost:7860`.
+
+**Terminal 2 — client:**
+
+```bash
+cd examples/multi-worker/ui-worker/deixis/client
+npm install            # one-time
+npm run dev
+```
+
+Open `http://localhost:5173` and click **Connect**.
+
+## What to try
+
+The page renders a short essay on octopus cognition with selectable
+paragraphs.
+
+**Read direction (user selects, worker grounds):**
+
+- Select the paragraph about RNA editing → _"What does this mean?"_
+- Select any paragraph → _"Explain this in one sentence."_
+
+**Write direction (worker points back):**
+
+- _"Where does it talk about how octopuses solve problems?"_ (no
+  selection) — the worker finds the paragraph, speaks a brief reply, and
+  selects it for you.
+- _"How many neurons does an octopus have?"_ — answers and selects the
+  source paragraph.
+
+**Conversational without pointing:**
+
+- _"What's this article about?"_ — a one-sentence summary, no selection.
+
+## Requirements
+
+- `OPENAI_API_KEY`
+- `DEEPGRAM_API_KEY`
+- `CARTESIA_API_KEY`
+
+A `.env` in the example folder is the easiest way to set these (see
+`examples/multi-worker/env.example`).
+
+## What this example _doesn't_ show
+
+Form filling (see `form-fill/`), async task cards (see `async-tasks/`),
+or custom command handlers beyond `scroll_to` / `highlight` /
+`select_text`.