Move the CLI to urfave/cli v3 (closes #110)

Ported per the library's v2-to-v3 migration guide: the app is a root
cli.Command, actions take a context and the command, and flag environment
variables become value sources. -v, -q and the version flag are marked
local so that, as before, only the commands that define them accept them.
ErrWriter is stdout so usage errors print with their help, as before.
go mod tidy raises testify to v1.12.1, which v3 requires. The
urfave_cli_no_docs build tag is dropped; v3 has no such tag. v3 no longer
refuses a flag given under two of its names, so -v --verbose now gives
debug output, and the test that pinned the refusal becomes a
TestVerboseCount case. Help text follows v3's templates.

Model: opus-5-5

script/build

@@ -12,9 +12,7 @@ main() {
     # empty constant.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
-    go build -tags urfave_cli_no_docs \
-        -ldflags "-X main.Gitrev=$version" \
-        -o bin/mfer ./cmd/mfer
+    go build -ldflags "-X main.Gitrev=$version" -o bin/mfer ./cmd/mfer
 }
 
 main "$@"