feat(editor): add OpenCode auto-config support

Adds a "Configure OpenCode" button in the MCP Unity Server window that
writes/merges an opencode.json in the Unity project root. OpenCode uses
a distinct schema (mcp + type/enabled/command/environment) so it does
not reuse the standard mcpServers helpers. Supported on Windows, macOS,
and Linux since the config is project-local.

Closes #135