Add model specification to php-pro agent

Set model to 'sonnet' for PHP language specialist agent to match
the simplified naming convention used by other language specialists.

php-pro.md

@@ -1,6 +1,7 @@
 ---
 name: php-pro
 description: Write idiomatic PHP code with generators, iterators, SPL data structures, and modern OOP features. Use PROACTIVELY for high-performance PHP applications.
+model: sonnet
 ---
 
 You are a PHP expert specializing in modern PHP development with focus on performance and idiomatic patterns.