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 prints "unknown",
the word script/build already uses when git describe gives nothing.

Model: opus-5-5

internal/cli/entry_test.go

@@ -11,6 +11,7 @@ import (
 	"net/http/httptest"
 	"os"
 	"path/filepath"
+	"runtime/debug"
 	"slices"
 	"strings"
 	"sync"
@@ -27,6 +28,7 @@ import (
 
 const (
 	testApp         = "mfer"
+	testGitrev      = "abc123"
 	testDir         = "/testdir"
 	testFile1       = "/testdir/file1.txt"
 	testMF          = "/testdir/test.mf"
@@ -93,8 +95,7 @@ func TestBuild(t *testing.T) {
 func testOpts(args []string, fs afero.Fs) *RunOptions {
 	return &RunOptions{
 		Appname: testApp,
-		Version: "1.0.0",
-		Gitrev:  "abc123",
+		Gitrev:  testGitrev,
 		Args:    args,
 		Stdin:   &bytes.Buffer{},
 		Stdout:  &bytes.Buffer{},
@@ -136,10 +137,24 @@ func TestVersionCommand(t *testing.T) {
 	exitCode := runCLI(opts)
 
 	assert.Equal(t, 0, exitCode)
+	assert.Equal(t, "mfer version "+testGitrev+"\n", testStdout(t, opts))
+}
 
-	stdout := testStdout(t, opts)
-	assert.Contains(t, stdout, mfer.Version)
-	assert.Contains(t, stdout, "abc123")
+// TestVersionFromBuildInfo covers a binary whose build stamped no Gitrev,
+// as `go install` builds it: mfer version prints the main module's version
+// from the build information instead.
+func TestVersionFromBuildInfo(t *testing.T) {
+	t.Parallel()
+
+	info, ok := debug.ReadBuildInfo()
+	require.True(t, ok)
+	require.NotEmpty(t, info.Main.Version)
+
+	opts := testOpts([]string{testApp, cmdVersion}, afero.NewMemMapFs())
+	opts.Gitrev = ""
+
+	require.Equal(t, 0, runCLI(opts))
+	assert.Equal(t, "mfer version "+info.Main.Version+"\n", testStdout(t, opts))
 }
 
 // TestVFlagCollision covers the -v/--verbose vs --version flag interaction
@@ -165,7 +180,7 @@ func TestVFlagCollision(t *testing.T) {
 			exitCode := runCLI(opts)
 
 			assert.Equal(t, 0, exitCode, "stderr: %s", testStderr(t, opts))
-			assert.Contains(t, testStdout(t, opts), mfer.Version)
+			assert.Contains(t, testStdout(t, opts), testGitrev)
 			assert.NotContains(t, testStderr(t, opts), "two forms of the same flag")
 		})
 	}
@@ -739,6 +754,9 @@ func TestBannerOutput(t *testing.T) {
 	stdout := testStdout(t, opts)
 	assert.Contains(t, stdout, "___")
 	assert.Contains(t, stdout, "\\")
+
+	// The banner shows the version mfer version prints, and nothing after it.
+	assert.Contains(t, stdout, "mfer by @sneak: "+testGitrev+"\n")
 }
 
 func TestUnknownCommand(t *testing.T) {