Copy go-bip39 into internal/bip39 (closes #122)

go-bip39's repository no longer exists. The part of v1.1.0 that secret
uses (NewEntropy, NewMnemonic, NewSeed, IsMnemonicValid and what they
call; English word list only) now lives in internal/bip39 with
upstream's LICENSE beside it, along with upstream's tests of that code
and their vectors unchanged. Every import moves there, and the module
leaves go.mod and go.sum. Beyond the trimming, only what the linter
asked for changed. No derived key or mnemonic changes.

Model: opus-5-5

TODO.md

@@ -18,6 +18,11 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-07: The part of `github.com/tyler-smith/go-bip39` v1.1.0 that `secret`
+  uses is copied into `internal/bip39`, with upstream's `LICENSE` beside it,
+  because its repository no longer exists
+  (https://git.eeqj.de/sneak/secret/issues/122). Every import moved there, and
+  the module left `go.mod` and `go.sum`. No derived key or mnemonic changes.
 - 2026-10-07: The canonical files are re-vendored from `sneak/prompts` commit
   `dd4027b` (https://git.eeqj.de/sneak/secret/issues/121), with golangci-lint
   v2.14.0 in the lint phase. Lint and test are phases of the `Dockerfile`, and