Copy go-bip39 into internal/bip39 (closes #42)

go-bip39's repository no longer exists, so keyfunc now carries the
part of v1.1.0 it uses, with the upstream LICENSE beside it, and
imports it from internal/bip39. Upstream's tests and test vectors for
the kept code come along unchanged; where they called a removed
function, crypto/rand stands in for NewEntropy and EntropyFromMnemonic
for MnemonicToByteArray.

Changes beyond the trimming are what the linter asked for: the
package-level variables moved into the functions that use them, three
error strings were lower-cased, and the unknown-word error now wraps
ErrInvalidMnemonic.

go.mod no longer requires go-bip39. go mod tidy keeps its two go.sum
lines, because a test in sneak/secret's bip85 package imports it, and
drops six lines that only go-bip39's own requirements needed.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

README.md

@@ -69,6 +69,8 @@ calls into `internal/`. The packages there are:
   `cli/options` holds the flags every command shares, `cli/signals` catches
   SIGINT, SIGTERM and SIGHUP for the commands that clean up before they end, and
   `cli/ssh`, `cli/age` and `cli/mnemonic` are the command groups.
+- `internal/bip39` is a copy of `github.com/tyler-smith/go-bip39` v1.1.0,
+  trimmed to what keyfunc uses.
 
 ### Adding a key type
 
@@ -373,9 +375,7 @@ standard: most Makefile targets are thin shims over an executable in `script/`
 
 ## TODO
 
-The open issues that stand between the tree and a 1.0 release:
-
-- [#42 go-bip39 no longer exists upstream: keep it, or copy it into the repo?](https://git.eeqj.de/sneak/keyfunc/issues/42)
+No issues are open.
 
 ## License