Derive flake version from git rev instead of hardcoding

Eliminates the manual version bump in flake.nix on each release.
Uses self.shortRev (or dirtyShortRev) since self.gitDescribe is not
yet available in this Nix version. Makefile builds still get the full
git describe output via ldflags.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

flake.nix

@@ -10,7 +10,7 @@
     let
       system = "x86_64-linux";
       pkgs = nixpkgs.legacyPackages.${system};
-      version = "0.7.5";
+      version = pkgs.lib.removePrefix "v" (self.gitDescribe or self.shortRev or self.dirtyShortRev or "unknown");
     in
     {
       packages.${system} = {