Add form-fill UIWorker example

A ReplyToolMixin UIWorker that fills inputs (fills) and toggles checkboxes /
presses submit (click) by voice — the state-changing half of the standard
action set.

examples/multi-worker/ui-worker/form-fill/README.md

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+# form-fill
+
+The UIWorker fills form inputs and clicks buttons by voice. The page
+renders a job application with text fields, a textarea, checkboxes, and
+a submit button. Tell the worker your name, email, and the rest; when
+you're ready, say "submit."
+
+## What it shows
+
+- The **state-changing actions**: `set_input_value` for writing into
+  inputs, `click` for checkboxes and submit. Both are bundled into the
+  same `ReplyToolMixin` that pointing and deixis use — `fills` is a list
+  of `{"ref", "value"}` so the LLM can fill several fields in one turn
+  ("my name is John Smith" fills first AND last name in one call), and
+  `click` is a list so checkboxes and submit run in order.
+- That `FormWorker` is a one-line composition:
+  `class FormWorker(ReplyToolMixin, UIWorker)`. Same shape as pointing
+  and deixis; the visual fields (`highlight`, `select_text`) just stay
+  `null` here, and the prompt steers the LLM toward `fills` / `click`.
+
+## What it adds vs. `pointing` and `deixis`
+
+Those exercise the visual / attention-pointing fields of `reply`. This
+one exercises the state-changing fields (`fills`, `click`). Same
+composition, same mixin — different fields per turn, driven by the
+prompt.
+
+## Run
+
+Two terminals.
+
+**Terminal 1 — bot:**
+
+```bash
+cd examples/multi-worker/ui-worker/form-fill
+uv run python bot.py
+```
+
+The bot starts on `http://localhost:7860`.
+
+**Terminal 2 — client:**
+
+```bash
+cd examples/multi-worker/ui-worker/form-fill/client
+npm install            # one-time
+npm run dev
+```
+
+Open `http://localhost:5173` and click **Connect**.
+
+## What to try
+
+- _"My name is John Smith."_ — fills first and last name in one call.
+- _"My email is john at gmail dot com."_ — converts the spoken form to
+  `mark@daily.co` and fills the email field.
+- _"I have five years of experience and I love working on real-time
+  voice agents."_ — fills two fields in one call.
+- _"Agree to the terms."_ — clicks the terms checkbox.
+- _"What have I entered so far?"_ — reads back current values from
+  `<ui_state>` (no fills, no clicks).
+- _"Submit it."_ — clicks submit. If terms isn't ticked yet, the worker
+  clicks both in order: terms, then submit.
+
+## 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
+
+Selection-based deixis (see `deixis/`) or async task cards (see
+`async-tasks/`).