Remove phpdbg from coverage target

[ohmyfelix]

Summary

Replace phpdbg with php in the coverage Makefile target so coverage runs use the standard PHP binary.

Motivation

This aligns the skeleton with the org-wide migration tracked in contributte/contributte#73.

Changes

• replace -p phpdbg with -p php in both branches of the coverage target in Makefile

Testing

• Tests pass locally
• CI passes
• Manual verification attempted: make coverage now invokes php, but this environment does not have Xdebug, PCOV, or PHPDBG available, so coverage cannot be generated locally