[codex] Mirror paid MCP answer-engine docs

[haochencheng]

Summary

Mirror the canonical paid-MCP answer-engine docs into the public Synapse-Network-Docs repository and update the GitHub/LLM indexes that answer engines can crawl.

This keeps the public docs funnel aligned with the canonical docs-front pages already merged in Synapse-Network:

• /mcp/paid-api-calls
• /mcp/synapse-vs-paid-mcp
• /mcp/brand-disambiguation

Why

The current Growing AI visibility evidence shows ChatGPT still prefers adjacent paid-MCP source hosts such as agent402.dev, docs.mcpay.tech, Paddle MCP/docs, mcpservers.org, x402, and other payment/API monetization pages. This PR makes the public GitHub docs mirror and llms.txt expose the same answer blocks and disambiguation terms as the canonical docs site.

Changes

• Add public mirror pages for paid API calls, paid MCP alternatives, and brand disambiguation.
• Add those pages to content/docs/mcp/meta.json.
• Update llms.txt with answer-engine hints, canonical SynapseNetworkAI surfaces, competitor/source-host terms, and brand-collision exclusions.
• Update README.md, README.zh-CN.md, and docs/mcp.md with direct links to the new pages.

Validation

• python3 -m json.tool content/docs/mcp/meta.json
• Canonical mirror compare for the three MDX pages and meta.json
• AI visibility term check for target queries and competitor/collision terms
• Secret-placeholder scan allowing only documented agt_xxx placeholders
• MCP meta file-existence check
• MDX frontmatter check
• git diff --check