README: policy sections and age and child mnemonic vectors (closes #21)

Add the sections REPO_POLICIES.md requires that the README lacked: a
first line naming category, license and author; Getting Started;
Entrypoints (one line per script/ file); Rationale; Design (the
internal/ packages, with the existing "Adding a key type" note moved
under it); TODO (the open issues between the tree and 1.0, as links);
License; and Author. Add the age recipients and identity vectors for
the eleven-abandon example mnemonic, copied from the agekey test, and
the BIP-85 specification's own 12-word child mnemonic vector, copied
from the childmnemonic test.

Documentation only; no code changes.

Model: opus-4-8

README.md

@@ -1,16 +1,73 @@
 # keyfunc
 
-`keyfunc` turns a BIP-39 mnemonic into key pairs that can be recreated from
-that mnemonic at any time. The same mnemonic, key type and index always give the
-same key.
+`keyfunc` is an MIT-licensed Go command-line tool by
+[@sneak](https://sneak.berlin) that turns a BIP-39 mnemonic into SSH keys, age
+identities and child mnemonics, each of which can be recreated from that
+mnemonic at any time. The same mnemonic, key type and index always give the same
+key.
 
 It uses the BIP-85 entropy deriver from `git.eeqj.de/sneak/secret/pkg/bip85` and
 takes the same steps as that repository's `agehd` package.
 
-Commands are grouped by what is derived: `keyfunc ssh ...` for ed25519 SSH
-keys, `keyfunc age ...` for age identities and for encrypting and decrypting
-with them, and `keyfunc mnemonic ...` for child mnemonics derived from the
-main one.
+Commands are grouped by what is derived: `keyfunc ssh ...` for ed25519 SSH keys,
+`keyfunc age ...` for age identities and for encrypting and decrypting with
+them, and `keyfunc mnemonic ...` for child mnemonics derived from the main one.
+
+## Getting Started
+
+Build from a clone and run the binary:
+
+```
+git clone git@git.eeqj.de:sneak/keyfunc.git
+cd keyfunc
+make build
+./keyfunc --version
+```
+
+`make build` produces `./keyfunc`. Every deriving command needs a mnemonic; see
+[Giving it the mnemonic](#giving-it-the-mnemonic) for where it is read from, then
+for example:
+
+```
+./keyfunc ssh pub -n 0 --mnemonic-command 'secret get foo'
+```
+
+## Rationale
+
+A key you can derive again never has to be backed up. One mnemonic, kept safe
+once, stands behind every key this tool produces: lose a laptop and the SSH key,
+the age identity and any child mnemonic on it come back from the mnemonic alone,
+at the same index, byte for byte. Nothing else has to be written down, copied
+between machines, or stored in a secret manager, because it can always be
+derived again.
+
+## Design
+
+The entry point is a thin `cmd/keyfunc/main.go` (what `make build` builds) that
+calls into `internal/`. The packages there are:
+
+- `internal/derive` turns a mnemonic into the 32 bytes a key is made from: it
+  walks BIP-39 seed, BIP-32 master key and BIP-85 entropy, and holds the shared
+  constants (the byte count and the largest key index).
+- `internal/mnemonic` finds the mnemonic to work from — a command, an
+  environment variable, or a terminal prompt — and refuses one that fails the
+  BIP-39 checksum.
+- `internal/sshkey` turns the derived bytes into an ed25519 SSH key
+  (`sshkey.go`) and serves that key from an in-process SSH agent on a private
+  unix socket, keeping it out of any file (`agent.go`).
+- `internal/agekey` turns the derived bytes into an age identity and encrypts
+  and decrypts with it.
+- `internal/childmnemonic` derives a child mnemonic from the main one using
+  BIP-85's own mnemonic application.
+- `internal/cli` builds the cobra command tree and runs it. Under it,
+  `cli/options` holds the flags every command shares, and `cli/ssh`, `cli/age`
+  and `cli/mnemonic` are the command groups.
+
+### Adding a key type
+
+Adding a key type is one package under `internal/` that turns the 32 derived
+bytes into that type's key, plus one cobra subcommand under `internal/cli/` that
+groups its commands.
 
 ## Derivation
 
@@ -151,6 +208,15 @@ the same steps `sneak/secret` takes in its `agehd` package. `secret` derives at
 a vendor-specific path today; for its keys to equal this tool's it moves to
 this path, which is a change in `secret`, not here.
 
+Test vectors, mnemonic
+`abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon abandon about`:
+
+```
+recipient index 0: age1xwdy9y6ckyfsgjc8k02e9uhsf3fmjy0ufysewlj68kmx5n67e3nsg2mftq
+recipient index 1: age1pmm92sxaf5mazjwvjph7dx2zq9r5p8l3rarfgqm7hmakqhvgyy4q5p3w7j
+identity index 0: AGE-SECRET-KEY-19QKK2P38598XLXMQFFU3P7J9PLDD7527T70JDHGDJ7AMNF3XT44S00JFU5
+```
+
 ### `keyfunc age pub`
 
 Prints the recipient, the `age1...` public key, on one line.
@@ -183,33 +249,67 @@ not through step 4). Default 12 words. A child mnemonic is a full mnemonic in
 its own right: it can seed another `keyfunc`, another wallet, or `secret`, and
 it never has to be written down, since it can be derived again.
 
-## Adding a key type
+Test vector: the child-mnemonic step is checked against BIP-85's own published
+vectors, which derive from the specification's master key
+`xprv9s21ZrQH143K2LBWUUQRFXhucrQqBpKdRRxNVq2zBqsx8HVqFk2uYo8kmbaLLHRdqtQpUm98uKfu3vca1LqdGhUtyoFnCNkfmXRyPXLjbKb`.
+At key index 0 the 12-word English child mnemonic is:
 
-Adding a key type is one package under `internal/` that turns the 32 derived
-bytes into that type's key, plus one cobra subcommand under `internal/cli/` that
-groups its commands.
+```
+girl mad pet galaxy egg matter matrix prison refuse sense ordinary nose
+```
 
 ## Errors
 
 Errors go to standard error and the exit status is 1, except for `ssh to`,
 which passes through `ssh`'s own exit status.
 
-## Building and running
+## Entrypoints
 
-```
-make build    # produces ./keyfunc
-make check    # fmt-check, lint (golangci-lint) and tests
-```
+The repo adheres to the
+[Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
+standard: each Makefile target is a thin shim over an executable in `script/`.
 
-Examples:
+- `script/bootstrap` installs everything needed to build and develop (git, make,
+  Go), idempotently, from nix, apt, brew or apk; it does not install the linter,
+  which only runs inside Docker.
+- `script/setup` prepares a fresh clone: it runs `bootstrap`, then installs the
+  git pre-commit hook.
+- `script/projectname` prints the project name; other scripts call it so they
+  stay identical across repos.
+- `script/test` runs `go vet` and then the test suite, rerunning verbosely if a
+  test fails.
+- `script/lint` runs the linter inside the image built from `Dockerfile.lint`
+  (which pins the linter by hash), so a complaint fails the build and leaves no
+  container behind.
+- `script/fmt` formats the Go source in place.
+- `script/fmt-check` checks that formatting without writing, failing if anything
+  is unformatted.
+- `script/check` runs `test`, `lint` and `fmt-check` and changes no files.
+- `script/docker` builds the Docker image tagged with the project name.
+- `script/cibuild` is the CI build the Gitea workflow calls: it runs the linter,
+  then `docker build`.
+- `script/precommit` is what the git pre-commit hook runs: `go mod tidy` and
+  `go fmt`, failing if `go.mod` or `go.sum` changed, then `check`.
+- `script/install-precommit` installs the git pre-commit hook that runs
+  `script/precommit`.
 
-```
-keyfunc ssh pub -n 3 --mnemonic-command 'secret get foo'
-keyfunc ssh priv -n 3 > ~/.ssh/id_bip85_3
-keyfunc ssh install -n 3 user@example.com
-keyfunc ssh to -n 3 user@example.com uptime
-keyfunc age pub -n 0
-keyfunc age encrypt -n 0 --armor -o notes.age notes.txt
-keyfunc age decrypt -n 0 notes.age
-keyfunc mnemonic -n 1 --words 24
-```
+## TODO
+
+The open issues that stand between the tree and a 1.0 release:
+
+- [#14 Choose a license and add LICENSE](https://git.eeqj.de/sneak/keyfunc/issues/14)
+- [#15 Decide the Go module path before 1.0](https://git.eeqj.de/sneak/keyfunc/issues/15)
+- [#16 Do not pass the mnemonic environment variables on to ssh and sftp](https://git.eeqj.de/sneak/keyfunc/issues/16)
+- [#17 Clean up the agent socket and working files when a signal ends the tool](https://git.eeqj.de/sneak/keyfunc/issues/17)
+- [#22 1.0 release readiness](https://git.eeqj.de/sneak/keyfunc/issues/22)
+
+## License
+
+MIT. The license is not yet settled on the tracker
+([#14](https://git.eeqj.de/sneak/keyfunc/issues/14)); MIT is the recommended
+option there, so this README names it and the `LICENSE` file is added when that
+issue is answered.
+
+## Author
+
+[@sneak](https://sneak.berlin).