feat: Add cost-aware parallel execution with execution_group support

- Schema: Add execution_group and task-level complexity fields
- Executor: Hybrid dependency analysis (explicit + file conflicts)
- Executor: Cost-based batching (MAX_BATCH_COST=8, Low=1/Medium=2/High=4)
- Executor: execution_group priority for explicit parallel grouping
- Planner: Add guidance for execution_group and complexity fields

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.claude/workflows/cli-templates/schemas/plan-json-schema.json

@@ -118,6 +118,15 @@
               "pattern": "^T[0-9]+$"
             },
             "description": "Task IDs this task depends on (e.g., ['T1', 'T2'])"
+          },
+          "execution_group": {
+            "type": ["string", "null"],
+            "description": "Parallel execution group ID. Tasks with same group ID run concurrently. null = use depends_on logic"
+          },
+          "complexity": {
+            "type": "string",
+            "enum": ["Low", "Medium", "High"],
+            "description": "Task complexity for workload balancing (Low=1, Medium=2, High=4 cost units)"
           }
         }
       },