feat: InternalLinkingTask — System Agent task for semantic internal cross-linking (#228)

[saraichinwag]

Summary

Adds InternalLinkingTask to the System Agent — AI-powered semantic internal cross-linking of posts. Follows the exact same architecture as AltTextTask.

Closes #228

What It Does

1. Finds related posts by shared taxonomy overlap (categories=1pt, tags=2pts)
2. Filters out already-linked posts
3. Sends post content + related post data to AI
4. AI weaves links semantically into existing sentences (no "Related Posts" section)
5. Validates links were actually inserted
6. Updates post content and tracks via _dm_internal_links post meta

Files Added

• inc/Engine/AI/System/Tasks/InternalLinkingTask.php — The task (extends SystemTask)
• inc/Abilities/InternalLinkingAbilities.php — Two abilities: datamachine/internal-linking + datamachine/diagnose-internal-links
• inc/Cli/Commands/LinksCommand.php — wp datamachine links crosslink + diagnose

Files Modified

• SystemAgentServiceProvider.php — registered task
• data-machine.php — registered abilities
• Cli/Bootstrap.php — registered CLI command

Usage

Ability

wp_get_ability('datamachine/internal-linking')->execute([
    'post_ids' => [1234, 5678],
    'links_per_post' => 3,
]);

// Or by category
wp_get_ability('datamachine/internal-linking')->execute([
    'category' => 'birds',
]);

CLI

wp datamachine links crosslink --post_id=1234
wp datamachine links crosslink --category=birds --dry-run
wp datamachine links crosslink --all --links-per-post=3
wp datamachine links diagnose

Architecture

Same pattern as AltTextTask:

• One post per job → Action Scheduler handles batching
• RequestBuilder::build() with system agent model
• Idempotent via _dm_internal_links post meta tracking
• force flag to re-process