Auto-attach r4subusability as a core package

[PawanRamaMali]

Summary

Currently core_packages() returns 6 packages. With r4subusability maturing, it should be added as a 7th auto-attached package in attach.R.

Tasks

• Add r4subusability to core_packages() vector in R/attach.R
• Update DESCRIPTION to add r4subusability to Imports (move from Suggests)
• Update tests in test-attach.R to expect 7 packages
• Update README package table (already shows it as auto-attached)
• Bump version to 0.2.0

Context

The README already lists r4subusability as auto-attached (Yes), but the code only attaches 6 packages. This should be aligned once r4subusability is on CRAN.