fix: inject agent identity into system prompt for built-in agents (#2964)

Built-in agents (Sisyphus, Hephaestus, Atlas) now include explicit
identity section in their system prompts, matching custom agent behavior.

12 agent identity tests pass, 266 total pass, tsc clean.

Closes #2964

src/agents/sisyphus/gpt-5-4.ts

@@ -28,6 +28,7 @@ import type {
   AvailableCategory,
 } from "../dynamic-agent-prompt-builder";
 import {
+  buildAgentIdentitySection,
   buildKeyTriggersSection,
   buildToolSelectionTable,
   buildExploreSection,
@@ -106,6 +107,11 @@ export function buildGpt54SisyphusPrompt(
     ? "YOUR TASK CREATION WOULD BE TRACKED BY HOOK([SYSTEM REMINDER - TASK CONTINUATION])"
     : "YOUR TODO CREATION WOULD BE TRACKED BY HOOK([SYSTEM REMINDER - TODO CONTINUATION])";
 
+  const agentIdentity = buildAgentIdentitySection(
+    "Sisyphus",
+    "Powerful AI Agent with orchestration capabilities from OhMyOpenCode",
+  );
+
   const identityBlock = `<identity>
 You are Sisyphus - an AI orchestrator from OhMyOpenCode.
 
@@ -421,7 +427,8 @@ If the user's approach has a problem, explain the concern directly and clearly,
 </verbosity_controls>
 </style>`;
 
-  return `${identityBlock}
+  return `${agentIdentity}
+${identityBlock}
 
 ${constraintsBlock}