docs: improve agent API guidance and no-key operation discoverability

[openoms]

Summary

• add an AI agent playbook with explicit fallback behavior: if unclear, check API reference first
• add a dedicated no-key operations page focused on relevant unauthenticated invoice/status and public query flows
• improve discoverability via sidebar and cross-links from auth and LLM reference pages
• add llms.txt with canonical sources and hard rules for AI agents

Details

• new: docs/api/agent-playbook.md
• new: docs/api/no-api-key-operations.md
• new: static/llms.txt
• updated: docs/api/auth.mdx
• updated: docs/api/llm-api-reference.md
• updated: sidebars.js

No-Key Operations Scope

• included invoice-on-behalf mutations
• included status/default-wallet queries
• included public queries: btcPriceList, businessMapMarkers
• excluded auth bootstrap mutations from this page (captchaCreateChallenge, captchaRequestAuthCode, userLogin)

Validation

• yarn build passes locally