feat(workflows): add full Workflow section — visual constructor, dashboard & execution engine
## New Feature: Workflow Builder & Execution Engine
### Database Schema (4 new tables)
- workflows: pipeline definitions with status (draft/active/paused/archived), tags, canvas metadata
- workflowNodes: agent/container/trigger/condition/output blocks with canvas positions
- workflowEdges: directional connections between nodes (source→target)
- workflowRuns: execution history with per-node status tracking & timing
### Backend (server/workflows.ts + 13 tRPC endpoints in routers.ts)
- Full CRUD for workflows, nodes, edges
- Atomic canvas save (nodes + edges in one mutation)
- BFS graph execution engine: walks from trigger nodes, executes agents/containers in order
- Single-node test execution for individual block testing
- Run management: start, cancel, poll status, list history
- Aggregated workflow stats (success rate, avg duration, run counts)
### Frontend — Visual Constructor
- WorkflowCanvas: interactive drag-and-drop builder with:
- Node palette sidebar (trigger/agent/container/condition/output types)
- Agent list for quick drag-to-canvas agent nodes
- Edge drawing between output→input ports with bezier curves
- Pan/zoom controls + grid background
- Keyboard shortcuts (Delete, Ctrl+S)
- Real-time run status overlays (running/success/failed per node)
- WorkflowNodeBlock: kind-aware visual cards with status indicators & connection ports
- WorkflowNodeEditModal: per-kind configuration (agent selector, Docker image/env, condition expressions, cron/webhook triggers)
- WorkflowCreateModal: create new workflows with name, description, tags
- WorkflowDashboard: monitoring panel with stats cards, run history timeline, per-node progress bars
- Workflows page: unified list/canvas/dashboard views with tabs
### Navigation & Routing
- Added Workflows nav item (GitBranch icon) in sidebar between Agents and Tools
- Routes: /workflows (list), /workflows/:id (dashboard+canvas)
### Also includes
- fix(nodes): keep AddNodeDialog open after join + canJoin guard
5ff2ade579