fix(model-fallback): align OpenAI fallback resolution across CLI and runtime

Keep install-time and runtime model tables in sync, stop OpenAI-only misrouting when OpenCode Go is present, and add valid OpenAI fallbacks for atlas, metis, and sisyphus-junior.

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

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

src/shared/model-requirements.test.ts

@@ -178,6 +178,13 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
     expect(primary.model).toBe("claude-opus-4-6")
     expect(primary.providers).toEqual(["anthropic", "github-copilot", "opencode"])
     expect(primary.variant).toBe("max")
+
+    const openAiFallback = metis.fallbackChain.find((entry) => entry.providers.includes("openai"))
+    expect(openAiFallback).toEqual({
+      providers: ["openai", "github-copilot", "opencode"],
+      model: "gpt-5.4",
+      variant: "high",
+    })
   })
 
   test("momus has valid fallbackChain with gpt-5.4 as primary", () => {
@@ -213,6 +220,32 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
     const secondary = atlas.fallbackChain[1]
     expect(secondary.model).toBe("kimi-k2.5")
     expect(secondary.providers[0]).toBe("opencode-go")
+
+    const tertiary = atlas.fallbackChain[2]
+    expect(tertiary).toEqual({
+      providers: ["openai", "github-copilot", "opencode"],
+      model: "gpt-5.4",
+      variant: "medium",
+    })
+  })
+
+  test("sisyphus-junior has an OpenAI fallback before big-pickle", () => {
+    // given - sisyphus-junior agent requirement
+    const sisyphusJunior = AGENT_MODEL_REQUIREMENTS["sisyphus-junior"]
+
+    // when - locating the OpenAI fallback entry
+    const openAiFallback = sisyphusJunior.fallbackChain.find((entry) => entry.providers.includes("openai"))
+    const openAiFallbackIndex = sisyphusJunior.fallbackChain.findIndex((entry) => entry.providers.includes("openai"))
+    const bigPickleIndex = sisyphusJunior.fallbackChain.findIndex((entry) => entry.model === "big-pickle")
+
+    // then
+    expect(openAiFallback).toEqual({
+      providers: ["openai", "github-copilot", "opencode"],
+      model: "gpt-5.4",
+      variant: "medium",
+    })
+    expect(openAiFallbackIndex).toBeGreaterThan(-1)
+    expect(bigPickleIndex).toBeGreaterThan(openAiFallbackIndex)
   })
 
   test("hephaestus supports openai, github-copilot, venice, and opencode providers", () => {

src/shared/model-requirements.ts

@@ -121,6 +121,11 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
         model: "claude-opus-4-6",
         variant: "max",
       },
+      {
+        providers: ["openai", "github-copilot", "opencode"],
+        model: "gpt-5.4",
+        variant: "high",
+      },
       { providers: ["opencode-go"], model: "glm-5" },
       { providers: ["kimi-for-coding"], model: "k2p5" },
     ],
@@ -149,12 +154,22 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
     fallbackChain: [
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-sonnet-4-6" },
       { providers: ["opencode-go"], model: "kimi-k2.5" },
+      {
+        providers: ["openai", "github-copilot", "opencode"],
+        model: "gpt-5.4",
+        variant: "medium",
+      },
     ],
   },
   "sisyphus-junior": {
     fallbackChain: [
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-sonnet-4-6" },
       { providers: ["opencode-go"], model: "kimi-k2.5" },
+      {
+        providers: ["openai", "github-copilot", "opencode"],
+        model: "gpt-5.4",
+        variant: "medium",
+      },
       { providers: ["opencode"], model: "big-pickle" },
     ],
   },