Move entrypoint to cmd/keyfunc

REPO_POLICIES.md keeps Go entrypoints under cmd/, one main.go per binary
whose body is a single call into internal/. Move the root main.go there
and point the Makefile build target at ./cmd/keyfunc; the binary is still
written to ./keyfunc. main.go is otherwise unchanged. (closes #20)

Model: opus-4-8

cmd/keyfunc/main.go

@@ -0,0 +1,12 @@
+// Command keyfunc derives key pairs from a BIP-39 mnemonic.
+package main
+
+import (
+	"os"
+
+	"git.eeqj.de/sneak/keyfunc/internal/cli"
+)
+
+func main() {
+	os.Exit(cli.Main())
+}