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

@@ -112,8 +112,8 @@ Goals, in order:
     again. `scan` is designed to be cronned; the reports run at any
     time against the last completed scan.
 4.  **Clean stream separation.** Everything on stdout is machine-readable
-    data. All progress, warnings, and summaries go to stderr. Never mix
-    them.
+    data. All progress, warnings, summaries, and help and usage text go
+    to stderr. Never mix them.
 
 ### Constraints
 
@@ -149,8 +149,16 @@ Three subcommands, all implemented:
 sfdupes scan [--workers N] [-x] PATH...
 sfdupes report > dupes.tsv
 sfdupes trees > dupetrees.tsv
+sfdupes --version
 ```
 
+`sfdupes --version` (or `-v`) prints one line, `sfdupes VERSION`, to
+stdout and exits 0, writing nothing to stderr. `VERSION` is stamped in
+at build time from `git describe` (the tag or short commit) by
+`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
 
 All three subcommands operate on a single SQLite database file: