Bring open-mcp-apps into deepseek-harness

Key features

• The browser half takes five slots dsh publishes for exactly this kind of extension: conversation.view for the Apps tab, conversation.input.dock for the same surface before a session has a view ring, sidebar.footer.action for the Apps section, shell.overlay for browsing, and tool.call.toolview for the inline takeover.
• It is a system-prompt section, not a message.
• The hero gives way in a container.
• Nothing is shadowed or replaced — every seat is an additive one.
• The host half exists because the same origin rule blocks the plugin's own React tree from reading the app registry.