Remove dead files.dat references from build config (closes #22)

files.dat was the pre-database on-disk scan format; nothing has
produced it since the persistent SQLite database landed. Drop the
stale references from Makefile clean, .gitignore and .dockerignore.
make clean still removes the binary, and .gitignore still covers the
actual database artifacts (*.sqlite, *.sqlite-shm, *.sqlite-wal). The
sole remaining files.dat mention is the historical entry in TODO.md.

Model: opus-4-8

.dockerignore

@@ -2,7 +2,6 @@
 .claude
 .DS_Store
 sfdupes
-files.dat
 *.log
 *.out
 *.test

.gitignore

@@ -27,7 +27,6 @@ node_modules/
 *.log
 
 # Local scan data
-files.dat
 *.sqlite
 *.sqlite-shm
 *.sqlite-wal

Makefile

@@ -46,4 +46,4 @@ hooks:
 	@script/install-precommit
 
 clean:
-	rm -f $(BINARY) files.dat
+	rm -f $(BINARY)

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- remove the dead `files.dat` references from `Makefile`, `.gitignore`
+  and `.dockerignore` (2026-09-21, branch `next`, closes
+  https://git.eeqj.de/sneak/sfdupes/issues/22)
+
 - fix the lint-image pin comments and `FROM` form in `Dockerfile` and
   `Dockerfile.lint` (2026-08-10, branch `next`, closes
   https://git.eeqj.de/sneak/sfdupes/issues/25): dropped the false