fetch: destination directory, skip files already present, save the manifest, require a signer (closes #101)

fetch takes --dest (default .) and writes every file there through the
existing symlink and hard-link guards, which now work relative to that
directory. A file already there with the listed size and hash is
skipped; a leftover temp file is still replaced. Once every file
verifies, the manifest is saved as index.mf through the same temp file
and rename, so check runs on the result; a manifest that lists index.mf
or its temp name at the top of the tree is refused, since saving would
replace that file. --require-signature is shared with check and
enforced through verifyRequiredSigner. Both refusals come before any
file is downloaded or anything is written.

Model: opus-5-5

internal/cli/check.go

@@ -319,7 +319,7 @@ func (mfa *CLIApp) checkManifestOperation(ctx *cli.Context) error {
 	}
 
 	// Check signature requirement
-	requiredSigner := ctx.String("require-signature")
+	requiredSigner := ctx.String(flagRequireSignature)
 	if requiredSigner != "" {
 		err = verifyRequiredSigner(ctx.Context, chk, requiredSigner)
 		if err != nil {