feat(flow-coordinator): add slashCommand/slashArgs support for unified workflow

- Add buildNodeInstruction() to construct instructions from slashCommand field
- Support slashArgs with {{variable}} interpolation
- Append additional instruction as context when slashCommand is set
- Update unified-workflow-spec.md with slashCommand/slashArgs documentation

This enables frontend orchestrator JSON to be directly consumed by
flow-coordinator skill in Claude client.

.claude/skills/flow-coordinator/spec/unified-workflow-spec.md

@@ -48,6 +48,10 @@ interface PromptTemplateNodeData {
   label: string;                 // Display label in editor
   instruction: string;           // Natural language instruction
 
+  // === Slash Command (optional, overrides instruction) ===
+  slashCommand?: string;         // Slash command name (e.g., "workflow:plan")
+  slashArgs?: string;            // Arguments for slash command (supports {{variable}})
+
   // === Data Flow ===
   outputName?: string;           // Name for output reference
   contextRefs?: string[];        // References to previous outputs
@@ -63,6 +67,10 @@ interface PromptTemplateNodeData {
 }
 ```
 
+**Instruction Resolution Priority**:
+1. If `slashCommand` is set: `/{slashCommand} {slashArgs}` + optional `instruction` as context
+2. Otherwise: `instruction` directly
+
 ### FlowEdge
 
 ```typescript