Use combined proton beamgas files for 50ms configurations

Replace 100 individual proton beamgas files with single combined files
for all 50ms machine runtime configurations. This simplifies the
configuration by using pre-combined files from the combined/ directory.

Changes:
- Each config now reads from 1 combined file instead of 100 separate files
- Combined files located at */combined/*_combined.root
- Total frequencies preserved (sum of all 100 individual frequencies)

Affected configurations:
- 5GeV x 41GeV: 100 files → 1 file (12.2 Hz)
- 5GeV x 100GeV: 100 files → 1 file (22.0 Hz)
- 10GeV x 100GeV: 100 files → 1 file (31.9 Hz)
- 10GeV x 275GeV: 100 files → 1 file (32.6 Hz)
- 18GeV x 275GeV: 100 files → 1 file (22.5 Hz)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: rahmans1