Fix the image block: disable wp.media, preview via object URL
The Image block was unusable: clicking "Media Library" threw
`can't access property "media", … is undefined`, and uploads logged
404 and 500 network errors. Three separate defects in the Gutenberg ↔
atproto bridge:
1. isolated-block-editor bundles @wordpress/editor's media hook, which
registers the @wordpress/media-utils MediaUpload (the wp.media
Backbone frame) on the editor.MediaUpload filter. SkyPress never
loads the wp.media global and never overrode the filter, so the
"Media Library" button crashed. SkyPress has no media library —
images upload to the PDS as blobs — so register a filter returning
an inert component, removing the button everywhere (placeholder,
Replace flow). The Upload/drop-zone path is unaffected.
2. mediaUpload handed the block `id: cid`, so it treated the CID as a
WP attachment and fetched /wp/v2/media/<cid> → 404. PDS blobs aren't
attachments; omit the id so the block treats it as a URL image.
3. Preview used the live getBlob URL, but a just-uploaded blob is
unreferenced (temporary on the PDS) until a record commits it, so
com.atproto.sync.getBlob 500s for it. Preview from a local object
URL instead; attachBlobRefs rewrites that transient blob: URL to the
canonical, portable getBlob URL at publish, keeping stored records
clean.
Decision 0006 and the SP3 spec are corrected to match the object-URL
preview strategy.