Surfaces v2: slotted schema, opinionated action rendering, design contract

apps/server/src/db/migrations/0044_agent-surfaces-v2.sql

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+-- Surface schema v2: slotted documents. The header (status + progress strip)
+-- and footer (document-level actions) live beside the block list. Existing
+-- rows keep schema_version 1 and render as a legacy notice in the sidebar;
+-- new writes stamp schema_version 2.
+ALTER TABLE agent_surfaces ADD COLUMN IF NOT EXISTS header JSONB;
+ALTER TABLE agent_surfaces ADD COLUMN IF NOT EXISTS footer JSONB;