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

README.md

@@ -153,9 +153,11 @@ sfdupes --version
 ```
 
 `sfdupes --version` (or `-v`) prints one line, `sfdupes VERSION`, to
-stdout and exits 0, writing nothing to stderr. `-h` or `--help`, alone
+stdout and exits 0, writing nothing to stderr. `VERSION` is stamped in
-or after a subcommand, prints the help text to stderr and exits 0,
+at build time from `git describe` (the tag or short commit) by
-writing nothing to stdout.
+`make build` and the `Dockerfile`, and is `dev` otherwise. `-h` or
+`--help`, alone or after a subcommand, prints the help text to stderr
+and exits 0, writing nothing to stdout.
 
 ### Database