Add optional stateless MCP server for the real browser

Expose a conservative browser-cli tool surface to MCP hosts without
duplicating browser-control logic: every tool is a thin adapter over the
existing Python SDK, and each call builds a fresh BrowserCLI so the real
browser stays the only owner of tab and navigation state.

The MCP process resolves BROWSER_CLI_PROFILE, BROWSER_CLI_REMOTE, and
BROWSER_CLI_KEY explicitly instead of passing None down, so a pinned server
targets one browser rather than fanning listing calls out across every
connected browser. Explicit tool arguments still win.

Tool names keep a browser_ prefix for hosts that expose raw MCP names, while
BROWSER_CLI_MCP_TOOL_PREFIX lets hosts that already namespace by server drop
it and avoid names like browser_cli_testing_browser_tabs_list.

JavaScript evaluation, raw commands, storage writes, and session import stay
unexposed, and Streamable HTTP refuses non-loopback binds because the endpoint
has no authentication of its own; remote browsers go through browser-cli's
authenticated remote transport instead.

MCP stays an optional extra, so normal installs are unaffected.

browser_cli/mcp/__init__.py

@@ -0,0 +1,5 @@
+"""Stateless Model Context Protocol adapter for browser-cli."""
+
+from browser_cli.mcp.server import create_server, main
+
+__all__ = ["create_server", "main"]