docs: a worked --json example for every verb

[yassiEmp]

What

Agents are told to use --json, but the docs show human output. A reference with one real --json payload per verb (snapshot, click, fill, wait, doctor, launch, browsers) saves every agent a round trip.

Fix

Add docs/json-reference.md: one fenced JSON block per verb, captured from a real run, with the update and via fields explained once at the top.

Acceptance

• Every verb in USAGE has an entry.
• Payloads are real captures, trimmed only for length.
• Linked from the README "Commands" section and from skill-data/core/SKILL.md.