Sync Bluesky replies as WordPress comments

[pfefferle]

Fixes #5

Proposed changes:

• Add Comment_Sync class that polls app.bsky.notification.listNotifications for reply notifications
• Insert replies as WordPress comments with AT Protocol metadata (_atmosphere_bsky_uri, _atmosphere_bsky_cid, _atmosphere_author_did)
• Support nested reply threading via comment_parent
• Deduplicate by checking existing comment meta before inserting
• Cache author profiles via transients to reduce API calls
• Register hourly WP-Cron event, cleaned up on deactivation

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Publish a post to Bluesky via ATmosphere
• Have someone reply to the post on Bluesky
• Wait for the hourly cron (or trigger manually via WP-CLI: wp cron event run atmosphere_sync_comments)
• Verify the reply appears as a WordPress comment with comment_type = atmosphere
• Verify nested replies set comment_parent correctly
• Verify duplicate replies are not imported twice

Changelog entry

• Automatically create a changelog entry from the details below.

Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Added - for new features
• Changed - for changes in existing functionality
• Deprecated - for soon-to-be removed features
• Removed - for now removed features
• Fixed - for any bug fixes
• Security - in case of vulnerabilities

Message

Sync Bluesky replies back as WordPress comments.