CLI: repeating `--input` only processes the last file

[Shinrai]

Problem

Passing --input more than once processes only the last value, and nothing says the earlier ones were dropped.

Reproduced on @cldmv/fix-headers@2.1.3 in a fresh git project with two header-less files:

npx fix-headers --input a.mjs --input b.mjs --verbose
# fix-headers complete: scanned=1, updated=1, dryRun=false
# updated: b.mjs

a.mjs is left untouched and scanned=1. The obvious way to stamp just the files a change touched quietly skips all but the last one, so a header can be missing from a commit with no warning.

Fix

Make --input repeatable and process the union of every value given, in any order, with duplicates dropped. Folders should work as well as single files. If repeating it is deliberately unsupported, reject repeated --input with a clear error instead of silently keeping the last one. Add a CLI test for two --input files.

[cldmv-bot]

Marked status: implemented — resolved by #125, merged into next. This issue closes automatically when that work ships to the default branch.