feat(workflows): visual DAG canvas editor, drop legacy linear steps

Replace the linear step-based workflow system with a graph-only DAG
engine using React Flow. Adds visual canvas with drag-and-drop node
types (agent, foreach, conditional, merge, trigger, end), node config
panel, and topological list view. Drops the `steps` table, old linear
runner, StepFormModal, and all step CRUD endpoints. Existing linear
steps were already migrated to graph nodes in a prior migration — this
commit drops that table and removes all dead code paths.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Juan Roldan

agents.config.json

@@ -3,7 +3,8 @@
     {
       "id": "test",
       "name": "demo-agent",
-      "model": "deepseek/deepseek-v3.2",
+      "model": "deepseek/deepseek-chat",
+      "provider": "openrouter",
       "workspaceDir": "./workspaces/test",
       "allowedTools": [
         "filesystem",