Sync opam package metadata including x-maintenance-intent
and external dependency specifications.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- Remove bushel_contact.ml, use Sortal_schema.Contact directly
- Add typed service_kind variants: ActivityPub (with Mastodon/Pixelfed/PeerTube),
Bluesky, Github, Git, Twitter, Photo, Custom
- Add convenience accessors: github_handle, twitter_handle, mastodon_handle,
bluesky_handle
- Add find_by_handle and lookup_by_name to Sortal_store
- Update bushel_loader to load contacts from Sortal XDG store
- Fix sortal sync to skip PNG conversion when PNG already exists

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Adds png_thumbnail_path to the top-level Sortal module, complementing
the existing thumbnail_path export. This allows library users to access
the PNG-converted version of thumbnails after running sync.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Adds a new `sortal thumbnail HANDLE` command that prints the absolute
path to a contact's thumbnail image. Returns exit code 0 if found,
or exit code 1 if the contact doesn't exist or has no thumbnail.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>