docs: add a manual FLUJO installation guide

### 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](https://github.com/flujo-app/flujo-mcp-client-validation/tree/main/github-mcp).

Start with CONTRIBUTING.md and the installation-guides index, then review the linked FLUJO validation materials for the tested remote setup, endpoint, and secret-header details. Add the focused manual guide and index link, documenting the scoped token and read-only limits; done means the documented handshake, tool discovery, and public-repository read are reproducible without claiming untested OAuth, local, or write support.

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