Take the version from the git tag, not a constant (closes #80)

`mfer version`, `mfer --version` and the banner print the revision the
build stamps into main.Gitrev or, when it stamped none (as after
`go install` of a tag), the main module's version from
runtime/debug.ReadBuildInfo. The banner no longer shows a release date.

mfer.Version and mfer.ReleaseDate go, with main.Version and the version
field and parameter in internal/cli that nothing read. A program using
the library reads the module version from its own build information.
The README says where the version comes from.

Judgement call: a binary with no build information, or none holding a
module version, prints "unknown", the word script/build already uses
when git describe gives nothing.

Model: opus-5-5

internal/cli/mfer.go

@@ -6,12 +6,12 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"runtime/debug"
 	"time"
 
 	"github.com/spf13/afero"
 	"github.com/urfave/cli/v3"
 	"sneak.berlin/go/mfer/internal/log"
-	"sneak.berlin/go/mfer/mfer"
 )
 
 // Command and flag names shared across command definitions and tests.
@@ -58,7 +58,6 @@ var errUnexpectedArgument = errors.New("unexpected argument")
 //nolint:revive // established name used throughout the codebase and tests
 type CLIApp struct {
 	appname     string
-	version     string
 	gitrev      string
 	startupTime time.Time
 	exitCode    int
@@ -87,13 +86,26 @@ const banner = `
    \  \:\        \  \:\      \  \::/       \  \:\
     \__\/         \__\/       \__\/         \__\/`
 
-// VersionString returns the version and git revision formatted for display.
+// VersionString returns the version mfer reports: the revision the build
+// stamped into main.Gitrev or, when it stamped none, as with `go install`,
+// the main module's version from the binary's build information.
 func (mfa *CLIApp) VersionString() string {
 	if mfa.gitrev != "" {
-		return fmt.Sprintf("%s (%s)", mfer.Version, mfa.gitrev)
+		return mfa.gitrev
 	}
 
-	return mfer.Version
+	return moduleVersion(debug.ReadBuildInfo())
+}
+
+// moduleVersion returns the main module's version from the binary's build
+// information, or "unknown" when the binary has none or it holds no version,
+// as in a build with GO111MODULE=off.
+func moduleVersion(info *debug.BuildInfo, ok bool) string {
+	if !ok || info.Main.Version == "" {
+		return "unknown"
+	}
+
+	return info.Main.Version
 }
 
 // printVersion writes the version line shared by the --version flag and the
@@ -106,8 +118,7 @@ func (mfa *CLIApp) printBanner() {
 	if log.GetLevel() <= log.InfoLevel {
 		_, _ = fmt.Fprintln(mfa.Stdout, banner)
 		_, _ = fmt.Fprintf(mfa.Stdout,
-			"        mfer by @sneak: v%s released %s\n",
-			mfer.Version, mfer.ReleaseDate)
+			"        mfer by @sneak: %s\n", mfa.VersionString())
 		_, _ = fmt.Fprintln(mfa.Stdout, "            https://sneak.berlin/go/mfer")
 	}
 }