feat(cli): implement output abstraction layer

packages/cli/src/output/yaml.ts

@@ -0,0 +1,35 @@
+/**
+ * YAML renderer for CLI output.
+ *
+ * Renders structured data as YAML for machine consumption and human readability.
+ */
+
+import YAML from 'yaml'
+import type { AnyCommandResult, OutputOptions } from './types.js'
+
+/** Render command result as YAML */
+export function renderYaml<T>(
+  result: AnyCommandResult<T>,
+  _options: OutputOptions
+): string {
+  const { schema } = result
+
+  // Apply custom serializer if provided
+  if (schema.serialize) {
+    if (result.type === 'list') {
+      const serialized = result.data.map((item) => schema.serialize!(item))
+      return YAML.stringify(serialized)
+    } else {
+      const serialized = schema.serialize(result.data)
+      return YAML.stringify(serialized)
+    }
+  }
+
+  return YAML.stringify(result.data)
+}
+
+/** Render a single item as YAML document (for streaming) */
+export function renderYamlDoc<T>(item: T, serialize?: (data: T) => unknown): string {
+  const output = serialize ? serialize(item) : item
+  return YAML.stringify(output)
+}