feat: add MCP server (scan-files, scan-diff, get-rules)

vibecheck can now run as an MCP server for AI coding agents.
3 tools: scan-files, scan-diff (unique, changed lines only), get-rules.
start via: vibecheck --mcp or node dist/mcp.js
also fixes CLI version constant (was 1.2.0, now 1.5.0)
adds zod as explicit dependency

Author: yuvrajangadsingh