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.go

@@ -26,8 +26,7 @@ func noColorEnvSet() bool {
 // Use DefaultRunOptions for standard CLI execution, or construct manually for testing.
 type RunOptions struct {
 	Appname string    // Application name displayed in help and version output
-	Version string    // Version string (typically set at build time)
-	Gitrev  string    // Git revision hash (typically set at build time)
+	Gitrev  string    // Revision stamped in at build time, or empty
 	Args    []string  // Command-line arguments (typically os.Args)
 	Stdin   io.Reader // Standard input stream
 	Stdout  io.Writer // Standard output stream
@@ -36,10 +35,9 @@ type RunOptions struct {
 }
 
 // DefaultRunOptions returns RunOptions configured for normal CLI execution.
-func DefaultRunOptions(appname, version, gitrev string) *RunOptions {
+func DefaultRunOptions(appname, gitrev string) *RunOptions {
 	return &RunOptions{
 		Appname: appname,
-		Version: version,
 		Gitrev:  gitrev,
 		Args:    os.Args,
 		Stdin:   os.Stdin,
@@ -50,15 +48,14 @@ func DefaultRunOptions(appname, version, gitrev string) *RunOptions {
 }
 
 // Run creates and runs the CLI application with default options.
-func Run(appname, version, gitrev string) int {
-	return RunWithOptions(DefaultRunOptions(appname, version, gitrev))
+func Run(appname, gitrev string) int {
+	return RunWithOptions(DefaultRunOptions(appname, gitrev))
 }
 
 // RunWithOptions creates and runs the CLI application with the given options.
 func RunWithOptions(opts *RunOptions) int {
 	m := &CLIApp{
 		appname:         opts.Appname,
-		version:         opts.Version,
 		gitrev:          opts.Gitrev,
 		exitCode:        0,
 		maxManifestSize: mfer.MaxManifestSize,