migrate the signup and onboarding profile writes to sdk actions

The post-signup and onboarding writes (setPersonalDetails, upsertProfile,
overwriteSavedFeeds, setInterestsPref) move onto sdk actions over the pds
client, and the starter-pack and contact-import reads move to the lex
clients. Every upsertProfile call now writes a lex blob directly, so the
toLegacyBlobRef bridge has no remaining callers and is deleted.

src/lib/api/legacy-blob.ts

@@ -1,16 +0,0 @@
-import {BlobRef} from '@atproto/api'
-import {type BlobRef as LexBlobRef} from '@atproto/lex'
-
-/**
- * Bridge a lex blob ref (the plain-JSON `{$type: 'blob', ref, mimeType, size}`
- * that {@link uploadBlob} now returns) back to the legacy `BlobRef` class
- * instance.
- *
- * Only needed where a blob is handed to a legacy agent write: the legacy
- * lexicon blob validator checks `value instanceof BlobRef`, so a plain lex
- * blob fails validation, and the legacy serializer would put the wrong shape
- * on the wire. Drop each call as its write moves to the lex client.
- */
-export function toLegacyBlobRef(blob: LexBlobRef): BlobRef {
-  return BlobRef.fromJsonRef(blob as Parameters<typeof BlobRef.fromJsonRef>[0])
-}