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

@@ -267,7 +267,8 @@ which passes through `ssh`'s own exit status.
 
 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/`.
+standard: most Makefile targets are thin shims over an executable in
+`script/` (`build` and `clean` are the exceptions).
 
 - `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,