feat: Image generation post-sideload modes (featured/attach/insert)

[saraichinwag]

Why

Currently datamachine/generate-image only sets the generated image as a featured image. If a post already has a featured image, the new image lands in the media library unattached. There's no way to add images into post content via the ability.

Proposal

Add a mode parameter to control post-sideload behavior:

Modes

• featured (default, current behavior) — Set as featured image. Skip if post already has one.
• insert — Sideload AND insert a <!-- wp:image --> block into post_content.

Placement Algorithm (position parameter)

• after_intro (default for insert) — After the first paragraph block
• before_heading — Before the next H2/H3 heading
• evenly — Distribute evenly through content (for multiple images, insert at equal intervals between blocks)
• end — Append at the bottom of post content
• index:N — Insert before block at index N (0-based)

Interface

// Single image insert
wp_get_ability('datamachine/generate-image')->execute([
    'prompt' => 'A colorful parrot in a rainforest',
    'post_id' => 1234,
    'mode' => 'insert',
    'position' => 'after_intro',
]);

// Featured image (default, existing behavior)
wp_get_ability('datamachine/generate-image')->execute([
    'prompt' => 'A colorful parrot in a rainforest',
    'post_id' => 1234,
    // mode defaults to 'featured'
]);

Implementation scope

1. ImageGenerationAbilities.php — Add mode and position to input schema, pass through to context
2. ImageGenerationTask.php — Add insertImageInContent() method alongside existing trySetFeaturedImage():
   • Parse post_content into Gutenberg blocks (parse_blocks())
   • Find insertion point based on position algorithm
   • Build <!-- wp:image --> block with attachment data, alt text, proper alignment
   • Serialize blocks back and wp_update_post()
3. Route by mode — After sideload: featured → trySetFeaturedImage(), insert → insertImageInContent()

Block format

<!-- wp:image {"id":123,"sizeSlug":"large","linkDestination":"none"} -->
<figure class="wp-block-image size-large"><img src="URL" alt="ALT" class="wp-image-123"/></figure>
<!-- /wp:image -->

Builds on #229/#234 (standalone post_id support).