[US-052] Add example config bundle as release assets

[sven1103-agent]

Description

Add example OpenCode config bundles as release assets to help users understand the installation process and bundle structure.

Scope

• Create example config bundles (e.g., opencode-examples.tar.gz)
• Include multiple preset examples (mixed, openai, etc.)
• Include proper opencode-bundle.manifest.json for each bundle
• Document the bundle structure in release notes

Why

• Helps users understand how bundles work
• Provides sensible defaults for new projects
• Teaches the bundle installation flow by example

Milestone

Go Migration