tb-scripts: add ,split-1G utility for splitting large files

Add a new utility script for splitting files into 1GB chunks:

,split-1G - File splitting wrapper
- Splits one or more files into 1GB chunks with numeric suffixes
- Uses split(1) with --bytes=1G for consistent chunk sizes
- Generates 5-digit numeric suffixes (.00000, .00001, etc) for up to 99,999 parts
- Outputs verbose progress information during splitting
- Fails fast with set -e to catch errors during multi-file operations
- Added to tb-scripts package with coreutils as runtime dependency

This is useful for preparing large files for upload to services with file
size limits, or for splitting backups/archives into manageable pieces.

Example usage:
  ,split-1G large-backup.tar.gz
  # Creates: large-backup.tar.gz.00000, large-backup.tar.gz.00001, ...

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

Co-Authored-By: Claude <noreply@anthropic.com>

pkgs/tb-scripts/split-1G

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+set -e
+
+for f in "$@"; do
+    split --verbose --suffix-length=5 --bytes=1G --numeric-suffixes "$f" "$f."
+done