mcp: add Model Context Protocol server support

Adds a new `mcp` run mode to start an MCP server over stdio.
Implements tools for navigation and JS evaluation, along with
resources for HTML and Markdown page content.

src/log.zig

@@ -38,6 +38,7 @@ pub const Scope = enum {
     not_implemented,
     telemetry,
     unknown_prop,
+    mcp,
 };
 
 const Opts = struct {