feat(01-01): scaffold Go project with types

- shell.nix with go and gopls for NixOS dev environment
- go.mod for github.com/pieMusic/vmux module
- types.go with Process, Session, SessionState (Working/NeedsInput/Idle/Unknown)

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

shell.nix

@@ -0,0 +1,4 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  buildInputs = with pkgs; [ go gopls ];
+}