docs: refresh the docs for gasless agents, Unified Balance and the diagrams

[JaDi03]

What changed

• How it works: new section on the owner's Unified Balance, with a diagram; the fixed rule and the cycle now match the code (hard stops HOLD and ASK, notes for the agent, payments followed and recorded before the agent acts).
• Architecture: the agent is a smart contract account whose gas Circle Gas Station pays; the hub and shops services are described; the dashboard's chain choice and the three-column console.
• Smart contracts: the pilot agent wallet is labelled as the one from before gasless agents.
• Home page: the testnet warning is gone; the "why it exists" list says what embco checks before it pays; emoji extension enabled so the card icons render.
• README: project structure updated (new agent modules, unified_balance/, settings.py, ShopFunding); a short list of the Circle tools embco uses.
• Diagrams: One cycle, The fixed rule, System overview and How a payment is accepted redrawn; At a glance and the Unified Balance diagram added, light and dark.