Print --version to stdout (closes #15)

Cobra's built-in version flag prints through the writer that carries
help and usage, which is stderr here. The root command now defines
its own -v/--version flag and prints one line, "sfdupes VERSION", to
stdout; a failed write is a fatal error (exit 1). Help and usage stay
on stderr. README documents --version and --help, their streams and
exit codes. Tests cover both flags and the failed write.

Model: opus-5-5

TODO.md

@@ -29,6 +29,9 @@
 
 # Completed Steps
 
+- `--version` prints `sfdupes VERSION` to stdout; README documents it and
+  `--help` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/15)
+
 - `scan` stops cleanly on `SIGINT` or `SIGTERM`: commits what it has
   hashed, deletes nothing more, exits 1 (2026-10-04,
   https://git.eeqj.de/sneak/sfdupes/issues/5)