Add git staged files utility functions (v2.2.6)

[AntonioCS]

Summary

• Add core/util/git.mk with utility functions for working with git staged files
• mb_staged_files - returns staged files, optionally filtered by extension
• mb_run_on_staged - runs command only if staged files exist (single git call)
• mb_git_available - checks git availability at load time
• Graceful handling when git is not installed

Usage

# Get staged PHP files
$(call mb_staged_files,php)

# Get all staged files
$(call mb_staged_files)

# Run phpstan only on staged PHP files
$(call mb_run_on_staged,php,vendor/bin/phpstan analyse)

# Run prettier on all staged files
$(call mb_run_on_staged,,prettier --write)

Test plan

• Tests added for all functions
• All 180 assertions passing

Trello: https://trello.com/c/BfsJm1JU