docs: add adapter development guide

[nextlevelshit]

No documentation exists for building a new first-class adapter. Adding a new adapter requires implementing AdapterRunner and adding a case to ResolveAdapter() in opencode.go.

The existing docs/examples/custom-adapter.md only covers the ProcessGroupRunner fallback.

Create a "Building a new adapter" guide covering:

• The AdapterRunner interface
• Streaming events
• Sandbox integration
• Testing patterns

[nextlevelshit]

Resolved by PR #544.