Switchyard v0.3.1

OpenCode integration

• Install with one opencode plugin @vepando/switchyard command.
• Register switchyard/auto automatically through the documented config hook.
• Close plugin-owned gateways cleanly when OpenCode disposes the plugin.
• Add regression and black-box coverage for OpenCode model discovery.

Package migration

• Adds a compatibility bridge for the historical @vepando/switchyard-opencode package; new installs should use @vepando/switchyard.