refactor: move V1 code into v1/ subdirectory

- git mv cmd/ internal/ pkg/ test/ go.mod go.sum Makefile build.sh docs/ v1/
- Create v1/README.md with V1 documentation
- Update root README for V1 + V2 structure
- V1 still builds (cd v1 && go build ./cmd/hostkeeper) and 105 tests pass
- Root is now clean for V2 development

v1/cmd/hostkeeper/main.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+var (
+	version   = "dev"
+	buildTime = "unknown"
+)
+
+func main() {
+	// SSH_ASKPASS support: hostkeeper askpass
+	// Called by the SSH_ASKPASS script we create for key passphrases.
+	if len(os.Args) == 2 && os.Args[1] == "askpass" {
+		fmt.Print(os.Getenv("HK_PASSPHRASE"))
+		return
+	}
+
+	if err := Execute(); err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+		os.Exit(1)
+	}
+}