AIAT agent runner downloads

These binaries run the fixed-output local test agent against the deployed gate.
Default gate server:
  https://aiat.southpiece.net

Optional environment overrides:
  GATE_SERVER_URL
  AGENT_INSTANCE_ID
  AGENT_DOMAIN_SLUG
  AGENT_CAPABILITIES
  AIAT_TEST_AGENT_RESPONSE
  AIAT_AGENT_POLL_INTERVAL_MS
  AIAT_AGENT_HEARTBEAT_INTERVAL_MS

macOS:
  open aiat-agent-runner-macos-arm64.dmg
  Drag "AIATAgentRunner.app" to Applications, then open it from Applications.

If macOS still blocks the first launch, open System Settings > Privacy & Security
and choose Open Anyway for AIAT Agent Runner.

Linux:
  chmod +x ./aiat-agent-runner-linux-x64
  ./aiat-agent-runner-linux-x64

Windows:
  aiat-agent-runner-windows-x64.exe