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. These results cover the original contribution commit above. On 2026-09-22, the branch was updated to 3592587; its diff remains 31 added Markdown lines across two files. Checks on the updated head await maintainer approval, and were not rerun in the cloud during this follow-up.

Docs

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

Prepared and tested with Codex assistance for the FLUJO project.

[flujo-app]

Updated against main at 3592587. The reviewed diff still contains only the 31 added Markdown lines in the installation guide and index. The description now explicitly separates the passing checks on f3a794f from the updated head. Could a maintainer approve the six waiting workflows and review the guide when convenient?