Refocus core package on buyer payment infrastructure

[Abiorh001]

Summary

• Remove seller/facilitator implementation, examples, scripts, and tests from the public omniclaw core package
• Keep core focused on buyer-side payment infrastructure and nanopayment client primitives
• Update README, API docs, CLI docs, production hardening docs, Docker config, and env examples to match the new separation

Verification

• uv run pytest tests -q
• 644 passed

Notes

• Hosted facilitator work remains local-only on private branch hosted-facilitator-private-alpha and is not included in this PR.