docs: add ChatGPT app MCP guidance

[huacnlee]

Summary

• Add Longbridge ChatGPT App guidance across the home page, MCP docs, Skill install docs, QA docs, and Longbridge Skill metadata.
• Update the Skill hero with a ChatGPT banner and third ChatGPT install tab while keeping the generic Copy command flow intact.
• Replace the old ChatGPT Developer Mode flow with the official ChatGPT Apps search, authorize, and @longbridge usage flow, including screenshots.

Test Plan

• git diff --check
• git diff --cached --check
• bun run build:release

Notes: build passes with the existing VitePress dynamic/static import and chunk-size warnings.