feat(dashboard-agent): a TranscriptStorage over the dashboard agent's message rows

The real-schema conformance target for the storage contract: one row per message in chat_messages, with the runtime's state and cursors in two new nullable columns on chats. Paging is by position in SQL. The agent itself still persists through its hooks.

Author: ericallam

internal-packages/dashboard-agent-db/drizzle/0006_wooden_shaman.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "trigger_dashboard_agent"."chats" ADD COLUMN "transcript_state" jsonb;--> statement-breakpoint
+ALTER TABLE "trigger_dashboard_agent"."chats" ADD COLUMN "transcript_cursors" jsonb;