chore: create fulll/gh-gcs repository

[shouze]

Context

Part of the gh-gcs epic (#93).

fulll/gh-gcs is the companion repository that serves as the distribution point for the gh extension. It contains no source code — only a README and the release workflow.

Goal

Create the fulll/gh-gcs repository with:

• Visibility: public
• Topic: gh-extension (required for discoverability in gh extension search)
• README.md: short description explaining it is a gh extension wrapper for github-code-search, with install and usage instructions
• .gitignore: ignoring any compiled binary accidentally dropped at the root

README content (minimum)

# gh-gcs

> GitHub Code Search as a `gh` extension — powered by [github-code-search](https://github.com/fulll/github-code-search)

## Install

\```bash
gh extension install fulll/gh-gcs
\```

## Usage

\```bash
gh gcs --org <org> "<query>"
\```

All options from `github-code-search query` are supported.

## Upgrade

\```bash
gh extension upgrade gh-gcs
\```

Acceptance criteria

• Repository fulll/gh-gcs is public and accessible
• Topic gh-extension is set
• gh extension install fulll/gh-gcs resolves the repository
• README documents install, basic usage, and upgrade