fix(db): build the 0255 output-name index CONCURRENTLY

workspace_files is an existing (hot) table, so the plain CREATE UNIQUE
INDEX that drizzle-kit generated would hold ACCESS EXCLUSIVE for the
whole build. Rewritten to the runner convention: COMMIT breakpoint,
lock_timeout 0, CREATE UNIQUE INDEX CONCURRENTLY IF NOT EXISTS, restore
lock_timeout (mirrors migration 0250). A pre-existing duplicate pair
fails the build into an INVALID index that migrate.ts WARNs about —
dedupe, drop, re-run.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: j15z

packages/db/migrations/0255_chat_output_display_name_unique.sql

@@ -1 +1,10 @@
-CREATE UNIQUE INDEX "workspace_files_chat_output_display_name_unique" ON "workspace_files" USING btree ("chat_id","display_name") WHERE "workspace_files"."context" = 'output' AND "workspace_files"."chat_id" IS NOT NULL;
+-- workspace_files is an existing table: build the new partial unique index CONCURRENTLY
+-- so the build never write-locks the relation (runner convention — plain CREATE INDEX
+-- takes ACCESS EXCLUSIVE). A pre-existing duplicate (chat_id, display_name) output pair
+-- (an artifact of the pre-index allocation race) fails the build and leaves an INVALID
+-- index that migrate.ts surfaces with a WARN — dedupe the rows, drop the invalid index,
+-- and re-run.
+COMMIT;--> statement-breakpoint
+SET lock_timeout = 0;--> statement-breakpoint
+CREATE UNIQUE INDEX CONCURRENTLY IF NOT EXISTS "workspace_files_chat_output_display_name_unique" ON "workspace_files" USING btree ("chat_id","display_name") WHERE "workspace_files"."context" = 'output' AND "workspace_files"."chat_id" IS NOT NULL;--> statement-breakpoint
+SET lock_timeout = '5s';