docs: add monetize inference guide

[bussyjd]

Summary

• Adds comprehensive guide for the sell-side monetize inference lifecycle at docs/guides/monetize-inference.md
• Covers seller setup, buyer flow, self-hosted facilitator testing with Anvil fork, lifecycle management, architecture deep-dive, and troubleshooting
• Extracted from the feat/secure-enclave-inference branch for early peer review

Content outline

Part	Topic
1	Seller setup — agent init, pricing, offer, reconciliation
2	Buyer flow — 402 challenge, x402 payment headers
3	Self-hosted facilitator with Anvil fork testing
4	Lifecycle management — stop, delete, status
Appendix	Architecture deep-dive, troubleshooting

Test plan

• Review guide for accuracy against current CLI commands
• Verify code snippets match actual output
• Check formatting renders correctly on GitHub