* Add --panel flag to new-split command
Allows splitting a specific panel without changing focus first.
Usage: cmuxterm new-split <direction> [--panel <id|index>]
Example: cmuxterm new-split down --panel 1
* Return new panel ID from new-split command
new-split now returns the UUID of the newly created panel, enabling
reliable chaining of split operations without index drift issues.
Before: OK
After: OK F2675177-3838-49AF-A1A0-1744C0048E99
Example workflow to create left + 2x2 grid on right:
RIGHT=$(cmuxterm new-split right | awk '{print $2}')
BOTTOM=$(cmuxterm new-split down --panel $RIGHT | awk '{print $2}')
cmuxterm new-split right --panel $RIGHT
cmuxterm new-split right --panel $BOTTOM
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