files.dat was the scan format before the SQLite database; nothing has
produced it since. Drop the stale references from the Makefile clean
target, .gitignore and .dockerignore. make clean still removes the
binary and .gitignore still covers the database files. The only
remaining mention is the historical entry in TODO.md.
Model: opus-4-8 (implementation); fable-5-1 (merge)
