docs: add FLUJO remote client guide

[flujo-app]

Closes #3247.

Summary

Add a manual FLUJO guide for the hosted GitHub MCP server's read-only repository toolset, with a secret Bearer header and a file-read verification step.

Why

FLUJO users need the executable/URL/header fields translated into its connection UI. This guide follows the scenario proposed in #3247 and is supported by an actual authenticated UI test.

What changed

• Add docs/installation-guides/install-flujo.md with the tested setup and precise authentication limits.
• Link it from the installation index and add a support row that leaves untested local and interactive-OAuth behavior unclaimed.

MCP impact

• No tool or API changes. Both changed files are Markdown client instructions.
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

No tools changed. In FLUJO 3.45.2, the handshake discovered 13 read-only repository tools; get_file_contents returned the exact public fastmcp/server.py fixture from flujo-app/flujo-mcp-client-validation.

Genuine screenshots and the authentication scope. The test used an authorized GitHub CLI OAuth access token supplied as a secret Bearer header. Interactive OAuth, client registration, PAT-specific behavior, local transport and writes were not tested.

Security / limits

• No security or limits impact
• Auth / permissions considered. The guide uses a masked secret header, a read-only endpoint and only required repository permissions; no token is included in the patch or evidence.
• Data exposure, filtering, or token/size limits considered. Validation read one public fixture and made no repository mutation.

Tool renaming

• I am renaming tools as part of this PR
• I am not renaming tools as part of this PR. No runtime source changed.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

On exact head f3a794f, go test -v ./..., script/lint (0 issues), UPDATE_TOOLSNAPS=true go test ./..., script/generate-docs and git diff --check all passed in an isolated cloud machine. Docs generation produced no Markdown changes. An unrelated final-newline change in an existing tool snapshot was restored; the checkout is clean. The exact script/test (go test -race ./...) also passed with CGO_ENABLED=1 after installing gcc/libc6-dev. All requested checks are complete.

Docs

• Not needed
• Updated (README / docs / examples). Added the guide and installation-index entry.

Prepared and tested with Codex assistance for the FLUJO project.