refactor(cli): Extract shared deck and frontmatter utils

Create deck-utils.ts and frontmatter/shared.ts to consolidate
duplicated logic. Rename ensure-deck-preview.ts to preview-deck.ts.

src/cli/tools/frontmatter/shared.ts

@@ -0,0 +1,48 @@
+/**
+ * Shared frontmatter utilities — used by read-frontmatter, write-frontmatter,
+ * card-crud, preview-deck, and generate-card-deck.
+ */
+
+import yaml from "js-yaml";
+import type { DeckFrontmatter } from "./read-frontmatter.js";
+
+/**
+ * Parse YAML frontmatter from a CSV file's raw content.
+ * Expects `---\n...\n---\n` delimited frontmatter at the top of the file.
+ */
+export function parseFrontMatter(content: string): {
+  frontmatter?: DeckFrontmatter;
+  csvContent: string;
+} {
+  const parts = content.trim().split(/(?:^|\n)---\s*\n/g);
+
+  if (parts.length !== 3 || parts[0] !== "") {
+    return { csvContent: content };
+  }
+
+  try {
+    const frontmatterStr = parts[1].trim();
+    const frontmatter = yaml.load(frontmatterStr) as
+      | DeckFrontmatter
+      | undefined;
+    const csvContent = parts.slice(2).join("---\n").trimStart();
+    return { frontmatter, csvContent };
+  } catch (error) {
+    console.warn("Failed to parse front matter:", error);
+    return { csvContent: content };
+  }
+}
+
+/**
+ * Serialize a DeckFrontmatter object to YAML frontmatter string.
+ */
+export function serializeFrontMatter(frontmatter: DeckFrontmatter): string {
+  const yamlStr = yaml.dump(frontmatter, {
+    indent: 2,
+    lineWidth: -1,
+    noRefs: true,
+    quotingType: '"',
+    forceQuotes: false,
+  });
+  return `---\n${yamlStr}---\n`;
+}