blog: add MCPX post on MCP gateway with Unix composability

New blog post covering why MCPX exists, focusing on Unix composability
as the key differentiator vs raw MCP -- data filtering with --pick,
@file input, and Unix pipelines as context window gates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Codestz