Add Antigravity (agy) as a third agent backend for gemini-* models

api/openapi.yaml

@@ -1298,7 +1298,7 @@ components:
             Where the stage's task branches from. parent (the default): the commit its one after stage finished at, or
             for a stage with none, where the workflow was started from. main: the project's default branch. Refused on
             a stage with more than one after stage, which always branches from the default branch.
-        model: { type: string, x-go-type-skip-optional-pointer: true, description: "The stage agent's model, which also picks its vendor: gpt-* models run on Codex, every other model on Claude Code. Required without agents, refused with them." }
+        model: { type: string, x-go-type-skip-optional-pointer: true, description: "The stage agent's model, which also picks its vendor: gpt-* models run on Codex, gemini-* models on Antigravity, every other model on Claude Code. Required without agents, refused with them." }
         prompt: { type: string, description: "The stage agent's instructions; with agents, the instructions every agent of the team gets before its own." }
         agents:
           type: array
@@ -1528,9 +1528,9 @@ components:
         task_id: { type: string, description: Empty for a lead session. }
         member: { type: string, description: "The agent's name in its task's team; empty on a task with one agent, and for leads and reviewers." }
         owner: { type: string, description: "The login whose lead agent this is; empty for a task's session." }
-        vendor: { type: string, enum: [claude, codex], description: "Follows from the model the session was created with: gpt-* models run on Codex, every other model on Claude Code." }
+        vendor: { type: string, enum: [claude, codex, antigravity], description: "Follows from the model the session was created with: gpt-* models run on Codex, gemini-* models on Antigravity, every other model on Claude Code." }
         model: { type: string, description: "Requested model; empty means the vendor's default. A session only switches between its own vendor's models." }
-        vendor_session_id: { type: string, description: "Claude Code's session id, chosen by byre; or the Codex thread id, empty until the session's first turn starts its thread." }
+        vendor_session_id: { type: string, description: "Claude Code's session id, chosen by byre; or the Codex thread id or Antigravity conversation id, empty until the session first starts." }
         state:
           type: string
           enum: [idle, running, completed]