feat: distribute github-code-search as a gh extension (gh-gcs)

[shouze]

Goal

Make github-code-search available as an official gh extension under the name gh-gcs, without rewriting or duplicating logic. All business logic stays in this repository; fulll/gh-gcs is a lightweight binary distributor only.

Expected result

gh extension install fulll/gh-gcs
gh gcs --org fulll "useEffect"        # → full TUI, same experience as today
gh extension upgrade gh-gcs           # → automatic updates

Scope

• Subcommand exposed via gh gcs: query only (interactive TUI)
• The upgrade and completions subcommands remain exclusive to the standalone binary
• No rewrite in Go — existing pre-compiled Bun binaries are reused as-is

Architecture

fulll/github-code-search   ← all logic, build, binaries
       │
       │  repository_dispatch after release
       ▼
fulll/gh-gcs               ← thin repo, 0 source code
       │  downloads + renames assets
       │  publishes a gh release with the gh naming convention
       ▼
  gh extension install fulll/gh-gcs

Asset naming convention

github-code-search asset	gh-gcs asset
github-code-search-linux-x64	gh-gcs-linux-amd64
github-code-search-linux-arm64	gh-gcs-linux-arm64
github-code-search-macos-x64	gh-gcs-darwin-amd64
github-code-search-macos-arm64	gh-gcs-darwin-arm64
github-code-search-windows-x64.exe	gh-gcs-windows-amd64.exe

Sub-issues

• Fallback token via gh auth token in src/api.ts
• argv[0] detection in github-code-search.ts (gh-gcs mode)
• Create the fulll/gh-gcs repository with README and gh-extension topic
• Release workflow in fulll/gh-gcs
• Automatic trigger from cd.yaml via repository_dispatch

Acceptance criteria

• gh extension install fulll/gh-gcs works on macOS arm64, macOS x64, Linux x64, Linux arm64, Windows x64
• gh gcs --org fulll "query" launches the TUI without needing to export GITHUB_TOKEN (when gh auth login has been done)
• Explicit GITHUB_TOKEN continues to work (backward compatibility)
• A release in github-code-search automatically triggers a release in gh-gcs
• All checks pass: bun test && bun run lint && bun run knip