feat(cli): detect gh-gcs argv[0] and default to query subcommand

[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.

Reference

• Creating GitHub CLI extensions — non-Go precompiled extension

How to bootstrap the repository

Use the --precompiled=other flag to scaffold the repo with the correct structure (workflow included):

gh extension create --precompiled=other gh-gcs
cd gh-gcs
gh repo create fulll/gh-gcs --source=. --public --push
gh repo edit fulll/gh-gcs --add-topic gh-extension

This generates:

• .github/workflows/release.yml — skeleton to adapt (see feat(ci): trigger gh-gcs release automatically from cd.yaml #96)
• script/build.sh — not needed for our use case (we reuse pre-compiled binaries), can be removed

Note: the gh naming convention for precompiled extension binaries is gh-gcs-<os>-<arch>[.exe], e.g. gh-gcs-darwin-arm64, gh-gcs-linux-amd64, gh-gcs-windows-amd64.exe. All target combinations recognized by gh are listed in the manager.go source.

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