docs: add a manual FLUJO installation guide

从 CONTRIBUTING.md 和 installation-guides 索引开始，然后查看链接的 FLUJO 验证材料，了解已测试的远程设置、endpoint 和 secret-header 详细信息。添加专门的手动指南和索引链接，记录受限 token 和只读限制；完成的标准是：文档中所述的握手、工具发现和公共仓库读取都可以复现，且不声称支持未经测试的 OAuth、本地或写入功能。

Describe the feature or problem you’d like to solve

The installation-guides index covers several graphical MCP hosts but does not include FLUJO. A FLUJO user who wants to search a public repository or read an issue needs to translate the generic configuration into its remote-URL and secret-header fields.

Proposed solution

Add a focused manual remote setup guide and link it from the installation index after validating it. The initial scope would use a suitably scoped PAT with the documented hosted endpoint and read-only mode, then verify tool discovery and a public-repository read. Local-server and OAuth support should only be added to the support matrix after separate verification; the guide should preserve the documented requirement for a registered app when using OAuth.

Additional context

FLUJO: https://github.com/mario-andreschak/FLUJO

This issue follows the request in CONTRIBUTING.md to outline the scenario before a PR. Validated on 2026-09-08 UTC: FLUJO 3.45.2 completed the remote read-only repository-toolset handshake, discovered 13 tools and successfully retrieved a public fixture with get_file_contents. This test used a user-authorized GitHub CLI OAuth access token supplied as a secret Bearer header. Interactive OAuth, client registration, PAT-specific behavior, local transport and writes remain untested. Prepared with Codex assistance.

Exact setup, authentication scope, genuine screenshots and returned file.