refactor(php-shim): introduce PhpClass for reporting PHP class names

Rust has no runtime class name, so `Command::get_class` existed purely to
let each command hand back its PHP class name, supplied through the
two-argument variant of `delegate_command_trait_impls_to_inner!` at the
impl site. Replace it with a general `PhpClass` trait plus an
`impl_php_class!` macro, so the name is stated once next to the type
definition and the mechanism is reusable outside commands.

`Command` gains `PhpClass` as a supertrait and drops `get_class`, and
`VcsDriverKind`'s hand-rolled `php_class_name` table moves onto the trait.
Behavior is unchanged: the same class-name strings are reported, and the
base command state still panics when asked for a name it cannot supply.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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