refactor: move version to Go source, extract in flake.nix

Define version once in cmd/oubliette/main.go and use builtins.match
in flake.nix to extract it. Add versioning guidelines to CLAUDE.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/oubliette/main.go

@@ -12,6 +12,8 @@ import (
 	"git.t-juice.club/torjus/oubliette/internal/server"
 )
 
+const Version = "0.1.0"
+
 func main() {
 	configPath := flag.String("config", "oubliette.toml", "path to config file")
 	flag.Parse()