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

mfer/constants.go

@@ -1,12 +1,6 @@
 package mfer
 
 const (
-	// Version is the current mfer release version.
-	Version = "0.1.0"
-
-	// ReleaseDate is the date on which Version was released.
-	ReleaseDate = "2025-12-17"
-
 	// MaxDecompressedSize is the maximum allowed size of decompressed manifest
 	// data (256 MiB). This prevents decompression bombs from consuming excessive
 	// memory.