feat: Add GitHub deployment artifacts (homebrew, npm) (gt-ufep6)

- Add .goreleaser.yml for multi-platform binary releases
- Add npm-package/ with postinstall binary download
- Add release.yml workflow for GoReleaser + npm publish + Homebrew tap

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

npm-package/README.md

@@ -0,0 +1,42 @@
+# @gastown/gt
+
+Gas Town CLI - multi-agent workspace manager for coordinating AI coding agents.
+
+## Installation
+
+```bash
+npm install -g @gastown/gt
+```
+
+This will download the appropriate native binary for your platform during installation.
+
+## Usage
+
+```bash
+# Check version
+gt version
+
+# Initialize a new town
+gt init
+
+# View status
+gt status
+
+# List rigs
+gt rigs
+```
+
+## Supported Platforms
+
+- macOS (Intel and Apple Silicon)
+- Linux (x64 and ARM64)
+- Windows (x64)
+
+## Manual Installation
+
+If npm installation fails, you can download binaries directly from:
+https://github.com/steveyegge/gastown/releases
+
+## License
+
+MIT