fix: include custom outputs in xlsx finalisation

[yell0w12]

Fixes #312.

This PR updates XLSX finalisation so custom outputs are copied into the output folder and listed in the generated description sheet, matching the behavior of JSON finalisation.

QA:

• Reproduced the bug locally before the fix: JSON copied the custom output, while XLSX created results.xlsx but did not copy or list the custom output.
• Confirmed the same reproduction case passes after the fix.
• Added regression coverage to test_finalise_excel.
• Ran pytest: 128 passed.
• Ran prek run -a: all checks passed.