feat(agents): add gpt-5.5 native sisyphus support

Route GPT-5.5 through the existing GPT-5.4-native Sisyphus and Hephaestus prompt family while keeping one shared model-family helper.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/hephaestus/agent.test.ts

@@ -23,6 +23,23 @@ describe("getHephaestusPromptSource", () => {
     expect(source3).toBe("gpt-5-4");
   });
 
+  test("returns 'gpt-5-4' for gpt-5.5 models", () => {
+    // given
+    const model1 = "openai/gpt-5.5";
+    const model2 = "openai/gpt-5-5";
+    const model3 = "github-copilot/gpt-5.5";
+
+    // when
+    const source1 = getHephaestusPromptSource(model1);
+    const source2 = getHephaestusPromptSource(model2);
+    const source3 = getHephaestusPromptSource(model3);
+
+    // then
+    expect(source1).toBe("gpt-5-4");
+    expect(source2).toBe("gpt-5-4");
+    expect(source3).toBe("gpt-5-4");
+  });
+
   test("returns 'gpt-5-3-codex' for GPT 5.3 Codex models", () => {
     // given
     const model1 = "openai/gpt-5.3-codex";
@@ -96,6 +113,19 @@ describe("getHephaestusPrompt", () => {
     expect(prompt).toContain("<tool_usage_rules>");
   });
 
+  test("GPT 5.5 model returns GPT-5.4 optimized prompt", () => {
+    // given
+    const model = "openai/gpt-5.5";
+
+    // when
+    const prompt = getHephaestusPrompt(model);
+
+    // then
+    expect(prompt).toContain("You build context by examining");
+    expect(prompt).toContain("Never chain together bash commands");
+    expect(prompt).toContain("<tool_usage_rules>");
+  });
+
   test("GPT 5.3-codex model returns GPT-5.3 prompt", () => {
     // given
     const model = "openai/gpt-5.3-codex";

src/agents/hephaestus/agent.ts

@@ -1,6 +1,6 @@
 import type { AgentConfig } from "@opencode-ai/sdk";
 import type { AgentMode, AgentPromptMetadata } from "../types";
-import { isGpt5_4Model, isGpt5_3CodexModel } from "../types";
+import { isGpt5_3CodexModel, isGptNativeSisyphusModel } from "../types";
 import type {
   AvailableAgent,
   AvailableTool,
@@ -21,7 +21,7 @@ export type HephaestusPromptSource = "gpt-5-4" | "gpt-5-3-codex" | "gpt";
 export function getHephaestusPromptSource(
   model?: string,
 ): HephaestusPromptSource {
-  if (model && isGpt5_4Model(model)) {
+  if (model && isGptNativeSisyphusModel(model)) {
     return "gpt-5-4";
   }
   if (model && isGpt5_3CodexModel(model)) {