fix: make workbench trace migration idempotent

internal/db/migrations/0001_cloud_core_skeleton.sql

@@ -391,23 +391,28 @@ INSERT INTO workbench_trace_events (
   updated_at
 )
 SELECT
-  id,
-  trace_id,
-  agent_session_id,
-  trace_id,
+  source_trace_events.id,
+  source_trace_events.trace_id,
+  source_trace_events.agent_session_id,
+  source_trace_events.trace_id,
   NULL,
-  0,
-  id,
-  0,
+  source_trace_events.durable_projection_seq,
+  source_trace_events.id,
+  source_trace_events.durable_projection_seq,
   'agent_trace_event',
   false,
   false,
-  COALESCE(NULLIF(event_json, ''), '{}'),
-  occurred_at,
-  occurred_at
-FROM agent_trace_events
-WHERE id IS NOT NULL AND trace_id IS NOT NULL
-ON CONFLICT (id) DO NOTHING;
+  COALESCE(NULLIF(source_trace_events.event_json, ''), '{}'),
+  source_trace_events.occurred_at,
+  source_trace_events.occurred_at
+FROM (
+  SELECT
+    agent_trace_events.*,
+    (ROW_NUMBER() OVER (PARTITION BY trace_id ORDER BY occurred_at, id))::integer AS durable_projection_seq
+  FROM agent_trace_events
+  WHERE id IS NOT NULL AND trace_id IS NOT NULL
+) AS source_trace_events
+ON CONFLICT DO NOTHING;
 
 INSERT INTO workbench_turns (
   turn_id,

internal/db/schema.test.ts

@@ -80,6 +80,10 @@ test("initial migration declares Workbench fact backfill sources", async () => {
 
   assert.match(sql, /ON CONFLICT \(session_id\) DO NOTHING/u);
   assert.match(sql, /ON CONFLICT \(trace_id\) DO NOTHING/u);
+  const traceBackfillMatch = sql.match(/INSERT INTO workbench_trace_events \([\s\S]*?ON CONFLICT DO NOTHING;/u);
+  assert.ok(traceBackfillMatch, "missing workbench_trace_events idempotent backfill");
+  assert.match(traceBackfillMatch[0], /ROW_NUMBER\(\) OVER \(PARTITION BY trace_id ORDER BY occurred_at, id\)/u);
+  assert.match(traceBackfillMatch[0], /durable_projection_seq/u);
 });
 
 test("protocol record guards catch schema drift before runtime writes", () => {