docs(migrations): NULL embeddings BEFORE the column-type alter

The Postgres recipe ordered ALTER COLUMN TYPE vector(N) before the
UPDATE that clears stale embeddings. pgvector refuses to cast existing
vectors across dimensions ('expected 1024 dimensions, not 1536'), so
the recipe as written aborts the transaction on any brain that has
embeddings — which is every brain doing this migration. Swap the steps:
NULLs cast fine.

docs/embedding-migrations.md

@@ -115,12 +115,17 @@ BEGIN;
 -- 1. Drop the HNSW index. It can't survive the column type change.
 DROP INDEX IF EXISTS idx_chunks_embedding;
 
--- 2. Alter the column type.
-ALTER TABLE content_chunks ALTER COLUMN embedding TYPE vector(<NEW_DIMS>);
-
--- 3. Clear stale embeddings so they don't survive into the new space.
+-- 2. Clear stale embeddings FIRST. This must happen BEFORE the column
+--    alter: pgvector refuses to cast existing vectors across dimensions
+--    ("expected <NEW_DIMS> dimensions, not <OLD_DIMS>"), so altering a
+--    column that still holds old-width vectors aborts the transaction.
+--    NULLs cast fine. (The old vectors are unusable in the new space
+--    anyway — wiping them is step 3 of the recipe's rationale.)
 UPDATE content_chunks SET embedding = NULL, embedded_at = NULL;
 
+-- 3. Alter the column type (all rows are NULL now, so the cast succeeds).
+ALTER TABLE content_chunks ALTER COLUMN embedding TYPE vector(<NEW_DIMS>);
+
 -- 4. Recreate the HNSW index ONLY IF dims <= 2000. Above that, leave it
 --    indexless and rely on exact scans (gbrain searchVector handles this
 --    automatically — search just gets slower, not broken).