Allow for more streamlined MCP / Connections addition to prompts

Currently to add an MCP server to chat one has to click through 5 times, more with limited tool selection.
It would be great to have some chat based UX where `@mcp` or `#mcp` or `` allows for adding MCP quickly, then perhaps from the dialog allow limiting tool selection.

The issue names no files or tests. Start by tracing the existing chat flow for adding an MCP server and its limited-tool selection dialog; done means a chat-based trigger can add MCP connections with fewer steps and still permit tool selection.

Written by the indexing model from the issue text.