fix(migrations): wrap entire pgvector migration in DO block guard

The ALTER TABLE and CREATE INDEX referenced vector(1536) outside the
exception-handling DO block, so when pgvector wasn't installed they
crashed the migration runner — blocking ALL E2E runs on main.

Fix: move all DDL inside the single DO block so the EXCEPTION handler
catches any pgvector-related failure and skips the entire migration.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

platform/migrations/031_memories_pgvector.up.sql

@@ -3,28 +3,29 @@
 -- Adds a dense-vector embedding column to agent_memories to power semantic
 -- (cosine-similarity) memory recall alongside the existing FTS path.
 --
--- Requires the pgvector Postgres extension. The DO block is a no-op guard:
--- if the extension is unavailable this migration exits early so a boot
--- without pgvector installed does not break the migration sweep.
+-- Requires the pgvector Postgres extension. The entire migration is wrapped
+-- in a single DO block so if pgvector is unavailable, ALL statements are
+-- skipped (not just CREATE EXTENSION). This prevents "type vector does not
+-- exist" errors on the ALTER TABLE / CREATE INDEX that follow.
 --
 -- Issue: #576
 
 DO $migrate$
 BEGIN
   CREATE EXTENSION IF NOT EXISTS vector;
+
+  -- Nullable: rows written before pgvector is active have NULL embedding and
+  -- are excluded from cosine-similarity queries automatically.
+  ALTER TABLE agent_memories ADD COLUMN IF NOT EXISTS embedding vector(1536);
+
+  -- ivfflat approximate nearest-neighbour index for cosine similarity.
+  -- lists=100 is a reasonable default for tables up to ~1M rows.
+  -- Partial index (WHERE embedding IS NOT NULL) keeps it lean — unembedded
+  -- rows are skipped entirely.
+  CREATE INDEX IF NOT EXISTS agent_memories_embedding_idx
+    ON agent_memories USING ivfflat (embedding vector_cosine_ops)
+    WHERE embedding IS NOT NULL;
+
 EXCEPTION WHEN OTHERS THEN
-  RAISE NOTICE 'pgvector not available on this Postgres instance — 031_memories_pgvector skipped';
-  RETURN;
+  RAISE NOTICE 'pgvector not available — 031_memories_pgvector skipped (%%)', SQLERRM;
 END $migrate$;
-
--- Nullable: rows written before pgvector is active have NULL embedding and
--- are excluded from cosine-similarity queries automatically.
-ALTER TABLE agent_memories ADD COLUMN IF NOT EXISTS embedding vector(1536);
-
--- ivfflat approximate nearest-neighbour index for cosine similarity.
--- lists=100 is a reasonable default for tables up to ~1M rows.
--- Partial index (WHERE embedding IS NOT NULL) keeps it lean — unembedded
--- rows are skipped entirely.
-CREATE INDEX IF NOT EXISTS agent_memories_embedding_idx
-  ON agent_memories USING ivfflat (embedding vector_cosine_ops)
-  WHERE embedding IS NOT NULL;