feat: add yaml language server support

[Sam-3Degrees]

Summary

• Add YAML language server support to opencode
• Enables syntax highlighting, error detection, and schema validation for .yml and .yaml files
• Uses yaml-language-server via Bun's package runner for automatic installation

Testing

• ✅ Verified LSP starts correctly: service=lsp.client serverID=yaml starting client and initialized
• ✅ Tested with multiple YAML file formats (.yml, .yaml)

Fixes

• Closes #1343