[CI] GitHub Action for building and releasing binaries

[randomm]

Goal

Automate binary builds and GitHub Releases so users can download pre-built binaries.

Workflow Design

Trigger

on:
  push:
    tags:
      - 'v*.*.*'

Build Matrix

Platform	Architecture	Binary Name
macOS	arm64	opencode-darwin-arm64
macOS	x64	opencode-darwin-x64
Linux	arm64	opencode-linux-arm64
Linux	x64	opencode-linux-x64
Windows	x64	opencode-windows-x64.exe

Steps

1. Checkout code
2. Setup Bun
3. Install dependencies
4. Run bun run build (or equivalent)
5. Create GitHub Release
6. Upload binaries as release assets

Release Format

• Tag: v1.0.0
• Release title: v1.0.0
• Assets: All platform binaries
• Auto-generate release notes from commits

Reference

The upstream publish.yml had similar logic but also published to npm/Tauri. We only need the binary build + GitHub Release part.

Acceptance Criteria

• Workflow triggers on version tags
• Builds for all 5 platform/arch combinations
• Creates GitHub Release automatically
• Binaries downloadable from Releases page
• Build completes in reasonable time (<10 min)