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

@@ -1,10 +1,11 @@
 # keyfunc
 
-`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.
+`keyfunc` is a Go command-line tool by [@sneak](https://sneak.berlin) — its
+license is not yet chosen
+([#14](https://git.eeqj.de/sneak/keyfunc/issues/14)) — 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.
@@ -310,10 +311,9 @@ The open issues that stand between the tree and a 1.0 release:
 
 ## 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.
+Not yet chosen. The license is the owner's decision, still open on the tracker
+([#14](https://git.eeqj.de/sneak/keyfunc/issues/14)); the `LICENSE` file is added
+when that issue is answered.
 
 ## Author