Removes testify + indirect deps from go.mod/go.sum; rewrites
assert/require calls across cmd/dev/*, cmd/setup/*, devkit/* _test.go
to stdlib t.Fatalf patterns. go vet clean. TestRunTestGen_Good fails
pre-existing (missing cmd/dev/pkg/ env) at dev tip — unrelated to
this PR.
Closes tasks.lthn.sh/view.php?id=754
Co-authored-by: Codex <noreply@openai.com>
Via-codex-lane: Cyclops-754 dispatch
