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

The tool derives ed25519 SSH keys from a BIP-39 mnemonic and prints
them, either as an authorized_keys line or as an unencrypted OpenSSH
private key. Both README test vectors are asserted in the tests.

The mnemonic is looked for in the order the README gives, and refused
when it fails its checksum or when there is nowhere left to look.

The repository standards come with it: the vendored linter
configuration and policies, the script/ entrypoints with a thin
Makefile, and a Gitea workflow. Linting happens only inside the image
built from Dockerfile.lint, which pins the linter by hash, so the root
Dockerfile runs the formatting check, the tests and the build, and
script/cibuild runs the linter before it.

Model: opus-5

internal/mnemonic/mnemonic.go

@@ -0,0 +1,117 @@
+// Package mnemonic finds the mnemonic the keys are derived from.
+package mnemonic
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+
+	bip39 "github.com/tyler-smith/go-bip39"
+	"golang.org/x/term"
+)
+
+const (
+	// CommandVariable holds a shell command whose output is the
+	// mnemonic.
+	CommandVariable = "KEYFUNC_MNEMONIC_COMMAND"
+
+	// Variable holds the mnemonic itself.
+	Variable = "KEYFUNC_MNEMONIC"
+)
+
+// ErrMissing is returned when there is nowhere left to look and
+// standard input is not a terminal, so there is nobody to ask.
+var ErrMissing = errors.New(
+	"no mnemonic given and standard input is not a terminal",
+)
+
+// ErrChecksum is returned for a mnemonic that fails the BIP-39
+// checksum.
+var ErrChecksum = errors.New("the mnemonic fails its BIP-39 checksum")
+
+// Read returns the mnemonic. The command given on the command line is
+// used first; then the command in KEYFUNC_MNEMONIC_COMMAND; then the
+// mnemonic in KEYFUNC_MNEMONIC; then a prompt on the terminal with
+// echo turned off. The first source that has one wins.
+func Read(ctx context.Context, command string) (string, error) {
+	if command == "" {
+		command = os.Getenv(CommandVariable)
+	}
+
+	if command != "" {
+		words, err := run(ctx, command)
+		if err != nil {
+			return "", err
+		}
+
+		return checked(words)
+	}
+
+	if words := os.Getenv(Variable); words != "" {
+		return checked(words)
+	}
+
+	return ask()
+}
+
+// run executes the command with sh and returns its standard output.
+// If the command fails, its standard error becomes part of the error.
+func run(ctx context.Context, command string) (string, error) {
+	//nolint:gosec // running the user's own command is the point
+	shell := exec.CommandContext(ctx, "sh", "-c", command)
+
+	var complaint bytes.Buffer
+
+	shell.Stderr = &complaint
+
+	out, err := shell.Output()
+	if err != nil {
+		said := strings.TrimSpace(complaint.String())
+		if said == "" {
+			return "", fmt.Errorf("the mnemonic command failed: %w", err)
+		}
+
+		return "", fmt.Errorf(
+			"the mnemonic command failed: %w: %s", err, said,
+		)
+	}
+
+	return string(out), nil
+}
+
+// ask prompts on the terminal with echo turned off.
+func ask() (string, error) {
+	fd := int(os.Stdin.Fd())
+
+	if !term.IsTerminal(fd) {
+		return "", ErrMissing
+	}
+
+	fmt.Fprint(os.Stderr, "mnemonic: ")
+
+	typed, err := term.ReadPassword(fd)
+
+	fmt.Fprintln(os.Stderr)
+
+	if err != nil {
+		return "", fmt.Errorf("reading the mnemonic: %w", err)
+	}
+
+	return checked(string(typed))
+}
+
+// checked drops the surrounding whitespace and refuses a mnemonic that
+// does not pass the BIP-39 checksum.
+func checked(words string) (string, error) {
+	words = strings.TrimSpace(words)
+
+	if !bip39.IsMnemonicValid(words) {
+		return "", ErrChecksum
+	}
+
+	return words, nil
+}