Replace bash parser with Python (shlex) for proper quote handling

[fkzys]

Replace the brittle bash-based config parser with a proper Python implementation using shlex for shell-style quote handling.

Key changes:

• lib/atomic/config.py – new Python parser with:

  • dump – JSON output
  • validate – check numeric values
  • shell – shell-safe KEY=value output
  • array – null-separated token array for CHROOT_COMMAND

• atomic-upgrade – uses config.py array to parse CHROOT_COMMAND with proper quote support

• common.sh – load_config() now consumes config.py shell output

• Tests – added test_config.py, removed obsolete test_config.sh

• CI & Makefile – updated to run Python tests, install config.py

Benefits:

• Arguments with spaces now work: CHROOT_COMMAND="pacman -S 'nvidia dkms'"
• No more quote stripping limitations
• Better validation and error messages