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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.70%. Comparing base (aa8cb42) to head (ad0b970).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #383   +/-   ##
=======================================
  Coverage   99.70%   99.70%           
=======================================
  Files           9        9           
  Lines        1354     1354           
=======================================
  Hits         1350     1350           
  Misses          4        4           

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[JessUWE]

Hello @yell0w12

thanks for doing this - much appreciated.

Please would you:

• add your name to the citation file as described in here:

https://github.com/AI-SDC/ACRO/blob/main/CONTRIBUTING.md
https://github.com/AI-SDC/ACRO/blob/main/CITATION.cff