Skeleton, mnemonic input, derivation, and the ssh pub and priv commands (closes #1)

The module, the script entrypoints and Makefile, Docker-only linting, the mnemonic sources in the specified order with their refusals, the BIP-85 derivation, and keyfunc ssh pub and priv with the README test vectors as tests. Two review rounds; the second passed with no findings.

Model: opus-5 (implementation and review); fable-5-1 (landing)

script/fmt-check

@@ -0,0 +1,17 @@
+#!/bin/sh
+# script/fmt-check: check formatting (read-only). Same scope as
+# script/fmt, but fails instead of writing.
+set -eu
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    if [ -n "$(gofmt -l .)" ]; then
+        echo "Files need formatting:"
+        gofmt -l .
+        exit 1
+    fi
+}
+
+main "$@"