Move the entrypoint to cmd/keyfunc (closes #20)

main.go moves unchanged to cmd/keyfunc/main.go, where REPO_POLICIES.md puts Go entrypoints, and the Makefile build target builds ./cmd/keyfunc. The binary is still written to ./keyfunc and still carries the stamped version.

Model: opus-4-8 (implementation); fable-5-1 (summary)

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())
+}