Fix module path and restore cmd/ omitted by gitignore

Rename module github.com/johnellis/stalwart-migrator ->
github.com/LINUXexpert-org/stalwart-migrator to match the repository
location, so the module resolves under `go get`.

The initial commit's .gitignore listed the compiled binary as a bare
`stalwart-migrate` pattern, which Git matches at any depth -- so it also
excluded the cmd/stalwart-migrate/ source directory, and the initial
commit shipped without the CLI entrypoint. Anchor the pattern to the
repo root as /stalwart-migrate and add the four missing files.

go build, go vet, and go test ./... all pass.

cmd/stalwart-migrate/main.go

@@ -0,0 +1,53 @@
+// Command stalwart-migrate drives an in-place Stalwart Mail Server upgrade
+// (0.15.5 -> latest) through preflight checks, a defense-in-depth backup,
+// a checkpointed migration, and post-migration validation, with rollback
+// available at every step. See ARCHITECTURE.md for the full design.
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		usage()
+		os.Exit(1)
+	}
+
+	var err error
+	switch os.Args[1] {
+	case "preflight":
+		err = runPreflight(os.Args[2:])
+	case "run":
+		err = runRun(os.Args[2:])
+	case "status":
+		err = runStatus(os.Args[2:])
+	case "rollback":
+		err = fmt.Errorf("not implemented yet: see internal/rollback")
+	case "confirm":
+		err = fmt.Errorf("not implemented yet: see internal/checkpoint")
+	case "report":
+		err = fmt.Errorf("not implemented yet: see internal/validate")
+	default:
+		usage()
+		os.Exit(1)
+	}
+
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "stalwart-migrate:", err)
+		os.Exit(1)
+	}
+}
+
+func usage() {
+	fmt.Fprintln(os.Stderr, `usage: stalwart-migrate <command> [flags]
+
+commands:
+  preflight   run read-only checks and print the migration plan
+  run         --dry-run: simulate and validate against a sandbox (real cutover isn't implemented yet)
+  status      show the state of an in-progress or completed run
+  rollback    restore the pre-migration backup for a given run
+  confirm     close the rollback window for a completed run
+  report      print the validation report for a run`)
+}