Feature: check_code tool to flag deprecated and anti-pattern Unity APIs with line numbers #39

Why it matters

deprecated_patterns.json already maps legacy APIs to replacements, but it is only used as a side-warning in lookup_api. Developers upgrading to Unity 6 (or AI agents reviewing code) want "scan this file and tell me what to change". That is concrete, high-value, and easy to demo.

Proposal

New MCP tool check_code(code | file_path):

• Regex/token match against deprecated_patterns.json plus new Unity 6 renames (velocity -> linearVelocity, drag -> linearDamping, FindObjectsOfType -> FindObjectsByType, CGPROGRAM in URP, Camera.main in Update, GetComponent in Update, GameObject.Find in hot paths, async void outside event handlers, new WaitForSeconds in loops).
• Returns line numbers, the replacement, and a reason, optionally with a suggested patch.
• Extend the data schema with pattern (regex), severity, autofix.

Stretch: ship the same patterns as a Roslyn analyzer package for Unity, so the checks run in the editor and CI too.

Done when

check_code flags every pattern in a fixture file with correct line numbers, and the performance-optimization and csharp rules point the agent at it.