Feature: Add Classic Editor & Standard HTML Support for Post Import/Export (PushMd)

Push_MD_Plugin から始め、Push_MD_Markdown_Consumer と Push_MD_Markdown_Producer を通るインポート／エクスポートの経路を追跡します。指定された WordPress のエディター検出 API と Push_MD_HTML_Converter の要件を確認します。ブロックエディターの投稿が Gutenberg の動作を維持し、Classic Editor と標準 HTML の投稿がメタデータと書式を含めて両方向に正しく変換されれば完了です。

Overview

Currently, Push MD assumes all WordPress posts use the Gutenberg Block Editor and wraps content in block comments (e.g., <!-- wp:paragraph -->). On sites or post types where the Classic Editor is active (or where Gutenberg is disabled), importing Markdown yields block-comment markup in plain HTML editors, while exporting classic HTML posts is broken and even simple headings and lists do not convert to clean Markdown.

I understand Gutenberg is much cleaner to implement and HTML can be messy. But a lot of people still use Classic Editor or hand coded HTML.

We should support Classic Editor and non-block post types natively during both import (Markdown → HTML) and export (HTML → Markdown).

Note: I've already implemented this and will be sending a PR soon. Creating this issue for reference.

Technical Plan

1. Editor Detection Strategy

• Introduce a helper method is_block_editor_enabled( $post_type ) in Push_MD_Plugin:
  • Check WordPress core's use_block_editor_for_post_type( $post_type ) when available.
  • Fall back to checking if register_block_type exists.
  • Provide a filter push_md_use_block_editor allowing developers to override the decision per post type.

2. Markdown Consumer Wrapper (Push_MD_Markdown_Consumer)

• Wrap WordPress\Markdown\MarkdownConsumer to handle classic editor post targets (because we may not want to modify the vendor components):
  • When block editor is enabled for the target post type: delegate directly to MarkdownConsumer to return Gutenberg block markup.
  • When block editor is disabled for the target post type:
    • Consume Markdown via MarkdownConsumer.
    • Strip Gutenberg block comment wrappers (e.g., <!-- wp:... --> and <!-- /wp:... -->).
    • Remove block-specific CSS classes (wp-block-*).
    • Normalize excess blank lines to output clean standard HTML suitable for the Classic Editor.

3. Markdown Producer Wrapper (Push_MD_Markdown_Producer)

• Wrap WordPress\Markdown\MarkdownProducer to handle posts without block markup (because we may not want to modify the vendor components):
  • Detect whether content contains Gutenberg blocks (has_blocks( $content ) or fallback delimiter check <!-- wp:).
  • Gutenberg content: Delegate to MarkdownProducer and normalize the output Markdown.
  • Classic/Standard HTML content: Extract post metadata into YAML frontmatter and convert standard HTML into clean Markdown.

4. HTML to Markdown Converter (Push_MD_HTML_Converter)

• Provide bidirectional conversion and normalization utilities for HTML/Markdown:
  • Convert standard HTML tags (headings, paragraphs, lists, links, blockquotes, inline formatting) into idiomatic Markdown.
  • Handle list items, line breaks, and paragraph spacing cleanly without redundant blank lines.
  • Provide normalization routines for produced Markdown output - for common quirks like extra spaces, non-breaking spaces etc.