docs(bankr): add optional ClawCredit + x402 integration guide

[stablegenius49]

Summary

This PR adds an optional ClawCredit integration path for agents that use Bankr and also call paid external x402 APIs.

What changed

• Added new reference doc: bankr/references/clawcredit-integration.md
  • system boundaries (Bankr Agent API / Bankr LLM credits / x402 + ClawCredit)
  • endpoint routing guidance (api.bankr.bot + llm.bankr.bot vs external x402 endpoints)
  • 402 troubleshooting split by source
  • minimal routing pseudo-code and operational checklist
• Updated bankr/SKILL.md
  • added an "Optional: Pair Bankr with ClawCredit for x402 services" section
  • linked to the new integration reference
• Updated bankr/references/error-handling.md
  • clarified HTTP 402 handling for Bankr endpoints vs external x402 endpoints
• Updated bankr/references/llm-gateway.md
  • added a note clarifying that non-Bankr x402 402 errors should follow the ClawCredit path

Why

Users increasingly run mixed workflows where Bankr handles on-chain actions and separate x402 services are used in the same agent. This PR documents a safe, explicit integration model and prevents confusing Bankr LLM-credit errors with x402 payment errors.

Scope

Documentation-only changes. No runtime behavior changes.