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

internal/cli/manifest_loader.go

@@ -9,7 +9,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/urfave/cli/v2"
+	"github.com/urfave/cli/v3"
 )
 
 // manifestFetchTimeout bounds HTTP requests made to fetch a manifest.
@@ -67,9 +67,9 @@ func (mfa *CLIApp) openManifestReader(pathOrURL string) (io.ReadCloser, error) {
 // resolveManifestArg resolves the manifest path from CLI arguments.
 // HTTP(S) URLs are returned as-is. Directories are searched for index.mf.
 // If no argument is given, the current directory is searched.
-func (mfa *CLIApp) resolveManifestArg(ctx *cli.Context) (string, error) {
-	if ctx.Args().Len() > 0 {
-		arg := ctx.Args().Get(0)
+func (mfa *CLIApp) resolveManifestArg(cmd *cli.Command) (string, error) {
+	if cmd.Args().Len() > 0 {
+		arg := cmd.Args().Get(0)
 		if isHTTPURL(arg) {
 			return arg, nil
 		}