Stamp the git tag or short commit into the binary (closes #4)

A plain `docker build .` now stamps bsdaily's version: the VERSION
build argument when one is given, otherwise `git describe --tags
--always` on the .git in the build context, as the canonical
Dockerfile does. The build fails if .git is there and the version
still comes out empty, dev or unknown. A host `make` build stamps the
same `git describe`, or dev. bsdaily logs the version on the first
line of every run and prints it with --version; a build that stamps
nothing, or an empty value, reports dev.

Judgement call: the build line also takes the canonical -trimpath and
-s -w.
One //nolint (gochecknoglobals): -X can only set a package-level
variable.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

cmd/bsdaily/main.go

@@ -20,7 +20,18 @@ var (
 	errFromAfterTo    = errors.New("is after --to")
 )
 
+// Version is the git tag or short commit, set at link time with
+// -X main.Version=... by the Dockerfile and the Makefile. A build that sets
+// nothing, or sets it empty, reports dev.
+//
+//nolint:gochecknoglobals // -X can only set a package-level variable
+var Version string
+
 func main() {
+	if Version == "" {
+		Version = "dev"
+	}
+
 	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
 		Level: slog.LevelInfo,
 	}))
@@ -31,8 +42,11 @@ func main() {
 	rootCmd := &cobra.Command{
 		Use:          "bsdaily",
 		Short:        "Extract a single day's data from the latest daily snapshot",
+		Version:      Version,
 		SilenceUsage: true,
 		RunE: func(_ *cobra.Command, _ []string) error {
+			slog.Info("starting", "version", Version)
+
 			targetDates, err := parseTargetDates(dateFlag, fromFlag, toFlag)
 			if err != nil {
 				return err