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)

Dockerfile.lint

@@ -0,0 +1,15 @@
+# The linter, pinned by hash, with this repository linted inside it.
+# Building this file is how linting happens; see script/lint. Nothing
+# lints on the host, so the answer is the same everywhere.
+
+# golangci/golangci-lint:v2.12.2, 2026-09-07
+FROM golangci/golangci-lint:v2.12.2@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240
+
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+RUN golangci-lint run --timeout 5m