Group 'name,sha256' rows; print groups with 2+ files as 'hash: f1,f2,...'.
Each line is `filename,sha256`. Group by hash. For every hash that appears **2+ times**, print `<hash>: <files>` where `<files>` is the matching filenames sorted ascending and joined with `,`. Print groups sorted by hash ascending.
Test input:
a.txt,abc
b.txt,def
c.txt,abc
d.txt,def
e.txt,xyz
Expected output:
abc: a.txt,c.txt
def: b.txt,d.txt
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