Avoid excessive (and incorrect) dct:modified changes

[dalito]

In voc4cat PR processing, voc4cat transform --prov-from-git updates dct:modified dates for ALL concepts based on git history, even when only a few concepts actually changed. This happens because:

1. The CI round-trip (xlsx -> ttl -> split -> merge) rewrites ALL concept files via voc4cat-merge (git merge-file --theirs), stripping per-concept dct:modified dates (since xlsx doesn't preserve them)
2. --prov-from-git then re-adds dates from git history for ALL files
3. Each CI commit becomes the "most recent commit" for all files, causing a cascading date shift on every CI run

Result: The PR nfdi4cat/voc4cat#255 had 540 out of 566 changed files with only a dct:modified date change -- pure noise that obscures the 26 real changes.