bip85ssh: add spec for deterministic SSH keys from BIP-85

README for a new tool that derives ed25519 SSH key pairs from a BIP-39
mnemonic or xprv, using pkg/bip85 from sneak/secret with the same steps
as agehd. Path m/83696968'/592366788'/1822331379'/n'. Four commands:
pub, priv, install, ssh. The README is the spec; the code follows in
later commits.

Model: fable-5-1

bip85ssh/README.md

@@ -0,0 +1,115 @@
+# bip85ssh
+
+`bip85ssh` turns a BIP-39 mnemonic (or a BIP-32 `xprv`) into SSH key pairs that
+can be recreated from that secret at any time. The same secret and the same
+index always give the same key. Only ed25519 keys are produced.
+
+It uses the BIP-85 entropy deriver from `git.eeqj.de/sneak/secret/pkg/bip85` and
+follows the same steps as that repository's `agehd` package.
+
+## Derivation
+
+The secret is turned into a key like this:
+
+1. mnemonic -> BIP-39 seed (empty passphrase), or the `xprv` is parsed directly;
+2. seed -> BIP-32 master key;
+3. master key -> 64 bytes of BIP-85 entropy at the path below;
+4. entropy -> BIP-85 DRNG (SHAKE256); read 32 bytes;
+5. those 32 bytes are the ed25519 seed.
+
+The path is:
+
+```
+m/83696968'/592366788'/1822331379'/<n>'
+```
+
+- `83696968` is the fixed BIP-85 purpose.
+- `592366788` is the vendor id, `sha256("berlin.sneak") & 0x7fffffff`, the same
+  one `agehd` uses.
+- `1822331379` is the application id, `sha256("bip85ssh") & 0x7fffffff`.
+- `n` is the key index: flag `--index` / `-n`, default `0`.
+
+Test vector, mnemonic
+`abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about`:
+
+```
+index 0: ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIA1esgfi4OeaywgKh0o5r/8lMOUlUD/N+YoAiC8SNEML
+index 1: ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIB26T7hdDuUF6wfTQ7NpIpyeTgGha4NlhQjaAhap5dqs
+```
+
+## Giving it the secret
+
+The secret is never taken as a command-line argument. It is looked for in this
+order; the first one found wins:
+
+1. `--mnemonic-file <path>` or `--xprv-file <path>`: the file holds the secret
+   on one line.
+2. Environment variable `BIP85SSH_MNEMONIC` or `BIP85SSH_XPRV`.
+3. A prompt on the terminal with echo turned off. The prompt accepts either a
+   mnemonic or an `xprv`; a value starting with `xprv` is treated as an `xprv`.
+
+If none of these is available and standard input is not a terminal, the tool
+refuses and exits with status 1.
+
+## Commands
+
+Every command takes `--index` / `-n` and the secret flags above, and has
+`--help`. `bip85ssh --version` prints the version set at build time.
+
+### `bip85ssh pub`
+
+Prints one `authorized_keys` line to standard output:
+
+```
+ssh-ed25519 <key> <comment>
+```
+
+The comment defaults to `bip85ssh/<n>`; change it with `--comment`.
+
+### `bip85ssh priv`
+
+Prints the unencrypted private key in OpenSSH format (the
+`-----BEGIN OPENSSH PRIVATE KEY-----` block that `ssh` reads) to standard output
+and nothing else, so it can be redirected into a file. The key's comment is the
+same as for `pub`.
+
+### `bip85ssh install <[user@]host> [-- ssh options...]`
+
+Runs the system `ssh` to the host and, on the host:
+
+- creates `~/.ssh` with mode `0700` if it is missing;
+- creates `~/.ssh/authorized_keys` with mode `0600` if it is missing;
+- appends the `pub` line only if an identical line is not already there.
+
+It then prints `added` or `already present`. How this `ssh` connection
+authenticates is up to the user's normal `ssh` setup (existing keys, agent,
+password). Anything after `--` is passed to `ssh` unchanged.
+
+### `bip85ssh ssh <host> [ssh arguments...]`
+
+Derives the key, serves it from an SSH agent that runs inside the tool on a unix
+socket in a new private `0700` temporary directory, then runs the system `ssh`
+with `-o IdentityAgent=<that socket>` followed by the host and all remaining
+arguments unchanged. The tool exits with `ssh`'s exit status and removes the
+socket and directory on the way out. The private key is never written to disk.
+
+## Errors
+
+Errors go to standard error and the exit status is 1, except for the `ssh`
+command, which passes through `ssh`'s own exit status.
+
+## Building and running
+
+```
+make build    # produces ./bip85ssh
+make check    # fmt-check, lint (golangci-lint) and tests
+```
+
+Examples:
+
+```
+bip85ssh pub -n 3
+bip85ssh priv > ~/.ssh/id_bip85_3
+bip85ssh install -n 3 user@example.com
+bip85ssh ssh -n 3 user@example.com uptime
+```