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/precommit

@@ -0,0 +1,19 @@
+#!/bin/sh
+# script/precommit: run by the git pre-commit hook; fails the commit if
+# checks fail. Go extras run first: go mod tidy and go fmt, failing the
+# commit if they change go.mod or go.sum.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    go mod tidy
+    go fmt ./...
+    git diff --exit-code -- go.mod go.sum ||
+        { echo "go mod tidy changed files; stage and retry" >&2; exit 1; }
+    "$SCRIPT_DIR/check"
+}
+
+main "$@"