Drive a real headless Chrome browser.

One browser instance is shared across calls in this session; pages persist
between calls, so you can navigate once and then click, fill, and read on
subsequent calls. Every navigation passes through the Neuron egress guard —
in NEURON_GUARD=strict mode, denylisted hosts are refused.

Actions:

- `navigate` (url): open a URL in a new tab and make it active. Returns the
  page title and readable text.
- `click` (selector): click an element by CSS selector.
- `fill` (selector, text): clear and type text into an input/textarea.
- `content`: return the active page's visible text (truncated).
- `screenshot` (path): save a PNG of the active page to an absolute path.
- `close`: close the active tab (or the whole browser with `all: true`).

Selectors are plain CSS (`#id`, `.class`, `a[href*="docs"]`). Prefer reading
`content` before clicking blind — know the page before you act on it.
