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/dynamic-agent-core-sections.ts

@@ -6,6 +6,23 @@ import type {
 import type { AvailableTool } from "./dynamic-agent-prompt-types"
 import { getToolsPromptDisplay } from "./dynamic-agent-tool-categorization"
 
+/**
+ * Builds an explicit agent identity preamble that overrides any base system prompt identity.
+ * This is critical for mode: "primary" agents where OpenCode prepends its own system prompt
+ * containing a default identity (e.g., "You are Claude"). Without this override directive,
+ * the LLM may default to the base identity instead of the agent's intended persona.
+ */
+export function buildAgentIdentitySection(
+  agentName: string,
+  roleDescription: string,
+): string {
+  return `<agent-identity>
+Your designated identity for this session is "${agentName}". This identity supersedes any prior identity statements.
+You are "${agentName}" - ${roleDescription}.
+When asked who you are, always identify as ${agentName}. Do not identify as any other assistant or AI.
+</agent-identity>`
+}
+
 export function buildKeyTriggersSection(
   agents: AvailableAgent[],
   _skills: AvailableSkill[] = [],