Add --flatten-properties option to content-first:export to expand nested front-matter keys

Problem/Motivation

When exporting content with drush content-first:export, the generated Markdown front matter uses nested YAML for structured metadata. For example, metatag values are exported as:

---
meta:
title: My page title
description: Lorem ipsum
---

Tools like Obsidian do not support nested YAML properties in front matter. They only read flat key/value pairs, so the nested structure is either ignored or causes unexpected behaviour.

Proposed resolution

Add a --flatten-properties boolean option to content-first:export. When set, all nested front-matter properties are flattened into a single level using - as the key separator. Any depth of nesting is handled recursively.

Before (drush cf:export):

---
meta:
title: My page title
description: Lorem ipsum
og:
image: /sites/default/files/hero.jpg
---
After (drush cf:export --flatten-properties):

---
meta-title: My page title
meta-description: Lorem ipsum
meta-og-image: /sites/default/files/hero.jpg
---

Implementation

RenderedContent::getMarkdown() gains an optional bool $flattenProperties = FALSE parameter, threaded through to attributesToMarkdown().
A new protected flattenAttributes(array $data, string $prefix, string $separator): array method handles the recursive flattening.
ContentFirstCommands::export() gains the --flatten-properties option and passes the flag to getMarkdown().
Backward compatible: the default value is FALSE, so existing behaviour is unchanged.

New Drush option:

--flatten-properties Flatten nested front-matter properties using a dash
separator. Tools like Obsidian do not support nested
YAML keys, so "meta: {title: foo}" becomes
"meta-title: foo". Applies to all levels of nesting.

API changes

RenderedContent::getMarkdown() accepts a new optional bool $flattenProperties = FALSE parameter. Callers that do not pass the argument are unaffected.