Run workflow stages with finish_stage handoffs and lead signals

api/openapi.yaml

@@ -1006,15 +1006,10 @@ components:
 
     Stage:
       type: object
-      required: [name, vendor, model, prompt]
+      required: [name, model, prompt]
       properties:
         name: { type: string, description: "One line of 1 to 100 characters, unique within the workflow." }
-        vendor:
-          type: string
-          enum: [claude, codex]
-          x-enum-varnames: [StageVendorClaude, StageVendorCodex]
-          description: Must be the vendor that runs model (gpt-* models run on Codex, every other model on Claude Code).
-        model: { type: string }
+        model: { type: string, description: "The stage agent's model, which also picks its vendor: gpt-* models run on Codex, every other model on Claude Code." }
         prompt: { type: string, description: "The stage agent's instructions." }
 
     TaskWorkflow:
@@ -1154,6 +1149,7 @@ components:
         created_at: { type: string, format: date-time }
         forked_from: { type: string, description: "A fork's first start resumes this vendor session, cut at forked_at, under a new vendor_session_id; absent for other sessions." }
         forked_at: { type: string, description: "With forked_from: the vendor's id of the last thing the fork keeps, a message for Claude Code, a turn for Codex." }
+        workflow_stage: { type: integer, description: "The stage (from 1) of the task's workflow this session runs; absent for a session outside a workflow." }
 
     SessionCreate:
       type: object