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/agent-identity.test.ts

@@ -0,0 +1,141 @@
+/// <reference types="bun-types" />
+
+import { describe, it, expect } from "bun:test"
+import { buildAgentIdentitySection } from "./dynamic-agent-core-sections"
+import { createSisyphusAgent } from "./sisyphus"
+import { createHephaestusAgent } from "./hephaestus"
+import { mergeAgentConfig } from "./builtin-agents/agent-overrides"
+
+describe("buildAgentIdentitySection", () => {
+  describe("#given an agent name and role description", () => {
+    describe("#when building the identity section", () => {
+      it("#then includes the agent name prominently", () => {
+        const result = buildAgentIdentitySection("Sisyphus", "Powerful AI orchestrator from OhMyOpenCode")
+
+        expect(result).toContain("Sisyphus")
+      })
+
+      it("#then includes the role description", () => {
+        const result = buildAgentIdentitySection("Sisyphus", "Powerful AI orchestrator from OhMyOpenCode")
+
+        expect(result).toContain("Powerful AI orchestrator from OhMyOpenCode")
+      })
+
+      it("#then wraps content in an identity XML tag", () => {
+        const result = buildAgentIdentitySection("Hephaestus", "Autonomous deep worker")
+
+        expect(result).toContain("<agent-identity>")
+        expect(result).toContain("</agent-identity>")
+      })
+
+      it("#then explicitly states this identity overrides any prior identity", () => {
+        const result = buildAgentIdentitySection("Sisyphus", "Powerful AI orchestrator from OhMyOpenCode")
+
+        expect(result).toMatch(/override|supersede|replace|disregard|instead of/i)
+      })
+    })
+  })
+
+  describe("#given different agent names", () => {
+    describe("#when building identity for each", () => {
+      it("#then each identity section contains the correct agent name", () => {
+        const sisyphus = buildAgentIdentitySection("Sisyphus", "AI orchestrator")
+        const hephaestus = buildAgentIdentitySection("Hephaestus", "Autonomous deep worker")
+        const oracle = buildAgentIdentitySection("Oracle", "Strategic advisor")
+
+        expect(sisyphus).toContain("Sisyphus")
+        expect(sisyphus).not.toContain("Hephaestus")
+        expect(hephaestus).toContain("Hephaestus")
+        expect(hephaestus).not.toContain("Sisyphus")
+        expect(oracle).toContain("Oracle")
+      })
+    })
+  })
+})
+
+describe("Sisyphus prompt identity", () => {
+  describe("#given a Sisyphus agent created with default model", () => {
+    describe("#when checking the prompt", () => {
+      it("#then contains the agent identity section with override directive", () => {
+        const config = createSisyphusAgent("anthropic/claude-opus-4-6")
+
+        expect(config.prompt).toContain("<agent-identity>")
+        expect(config.prompt).toContain("Sisyphus")
+        expect(config.prompt).toContain("</agent-identity>")
+      })
+
+      it("#then identity section appears before the Role section", () => {
+        const config = createSisyphusAgent("anthropic/claude-opus-4-6")
+        const prompt = config.prompt ?? ""
+        const identityIndex = prompt.indexOf("<agent-identity>")
+        const roleIndex = prompt.indexOf("<Role>")
+
+        expect(identityIndex).toBeGreaterThanOrEqual(0)
+        expect(roleIndex).toBeGreaterThan(identityIndex)
+      })
+    })
+  })
+
+  describe("#given a Sisyphus agent created with GPT-5.4 model", () => {
+    describe("#when checking the prompt", () => {
+      it("#then contains the agent identity section", () => {
+        const config = createSisyphusAgent("openai/gpt-5.4")
+
+        expect(config.prompt).toContain("<agent-identity>")
+        expect(config.prompt).toContain("Sisyphus")
+        expect(config.prompt).toContain("</agent-identity>")
+      })
+    })
+  })
+})
+
+describe("Hephaestus prompt identity", () => {
+  describe("#given a Hephaestus agent created with GPT model", () => {
+    describe("#when checking the prompt", () => {
+      it("#then contains the agent identity section", () => {
+        const config = createHephaestusAgent("openai/gpt-5.4")
+
+        expect(config.prompt).toContain("<agent-identity>")
+        expect(config.prompt).toContain("Hephaestus")
+        expect(config.prompt).toContain("</agent-identity>")
+      })
+
+      it("#then identity section appears at the start of the prompt", () => {
+        const config = createHephaestusAgent("openai/gpt-5.4")
+        const prompt = config.prompt ?? ""
+        const identityIndex = prompt.indexOf("<agent-identity>")
+
+        expect(identityIndex).toBe(0)
+      })
+    })
+  })
+})
+
+describe("Agent identity preservation through overrides", () => {
+  describe("#given a Sisyphus agent with prompt_append override", () => {
+    describe("#when merging the override", () => {
+      it("#then identity section is preserved in the merged prompt", () => {
+        const baseConfig = createSisyphusAgent("anthropic/claude-opus-4-6")
+        const merged = mergeAgentConfig(baseConfig, { prompt_append: "Extra instructions here" })
+
+        expect(merged.prompt).toContain("<agent-identity>")
+        expect(merged.prompt).toContain("Sisyphus")
+        expect(merged.prompt).toContain("</agent-identity>")
+        expect(merged.prompt).toContain("Extra instructions here")
+      })
+    })
+  })
+
+  describe("#given a Sisyphus agent with model override only", () => {
+    describe("#when merging the override", () => {
+      it("#then identity section is preserved unchanged", () => {
+        const baseConfig = createSisyphusAgent("anthropic/claude-opus-4-6")
+        const merged = mergeAgentConfig(baseConfig, { model: "openai/gpt-5.4" })
+
+        expect(merged.prompt).toContain("<agent-identity>")
+        expect(merged.prompt).toContain("Sisyphus")
+        expect(merged.prompt).toContain("</agent-identity>")
+      })
+    })
+  })
+})