Resolve check and freshen paths against the manifest's directory (closes #177)

Without --base, check and freshen now look for a manifest's files in the
directory that holds the manifest, the file named or the one found in a
directory argument, instead of the current directory. check of a manifest
given by URL still uses the current directory, and a --base given on the
command line, the current directory included, still overrides. The --base
help text of both commands and the README's Tool Examples state the
default; the README gains a freshen entry to do so.

Model: opus-5-5

internal/cli/check.go

@@ -299,6 +299,17 @@ func (mfa *CLIApp) checkManifestOperation(
 		return fmt.Errorf("check: %w", err)
 	}
 
+	// Without --base, the manifest's paths are resolved against the
+	// directory holding it, or the current directory for a manifest URL.
+	basePath := "."
+
+	switch {
+	case cmd.IsSet(flagBase):
+		basePath = cmd.String(flagBase)
+	case !isHTTPURL(manifestPath):
+		basePath = filepath.Dir(manifestPath)
+	}
+
 	// URL manifests need to be downloaded to a temp file for the checker
 	if isHTTPURL(manifestPath) {
 		tmpPath, tmpErr := mfa.fetchManifestToTemp(ctx, manifestPath)
@@ -311,7 +322,6 @@ func (mfa *CLIApp) checkManifestOperation(
 		manifestPath = tmpPath
 	}
 
-	basePath := cmd.String("base")
 	showProgress := cmd.Bool("progress")
 
 	log.Infof("checking manifest %s with base %s", manifestPath, basePath)