Add minimum Claude Code version check

[bguidolim]

Problem

MCS increasingly relies on newer Claude Code features (e.g., hook timeout, async, statusMessage). When users run mcs sync with an outdated Claude Code installation, things silently break or produce confusing errors.

Proposal

Add a single minClaudeCodeVersion constant and check it at sync time:

1. Constant in Constants.swift — the minimum supported Claude Code version
2. Detection — run claude --version and parse the output
3. Sync gate — fail early with a clear "please upgrade Claude Code" message if below minimum
4. Doctor check — surface it as a core health check in mcs doctor

Why not per-feature mapping?

A per-feature version map would create maintenance burden for both mcs and tech pack authors, and the user action is always the same: upgrade Claude Code. A single minimum version is sufficient.

Prior art

This follows the same pattern as minMCSVersion in techpack.yaml.