Improve buyer server rail configuration and DX

[Abiorh001]

Summary

• split stable buyer policy from generated wallet runtime state and remove checked-in generated buyer policy
• add buyer rail modes/requirements for Circle transfer, Gateway, and x402 exact, with server/policy enforcement across pay, simulate, intents, can-pay, deposit, and withdraw paths
• improve CLI and docs/examples for x402 max amount, visible runtime policy, owner-auth paths, and Gateway funding commands
• add regression coverage for policy state separation, x402-only runtime, route rail checks, and fail-closed intent confirmation

Verification

• uv run pre-commit run --all-files
• uv run pytest -q
• docker compose -f examples/agent/buyer/docker-compose.yml config
• docker compose -f examples/agent/docker-compose.yml config