fix(models): remove stale Gemini 3 Pro references

Keep repo-owned CLI, docs, and test fixtures aligned with current Gemini 3.1 naming while leaving upstream catalog behavior untouched.

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

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

src/hooks/model-fallback/hook.test.ts

@@ -337,7 +337,7 @@ describe("model fallback hook", () => {
     clearPendingModelFallback(sessionID)
   })
 
-  test("transforms model names for google provider via fallback chain", async () => {
+  test("preserves canonical google preview model names via fallback chain", async () => {
     //#given
     const sessionID = "ses_model_fallback_google"
     clearPendingModelFallback(sessionID)
@@ -351,20 +351,20 @@ describe("model fallback hook", () => {
 
     // Set a custom fallback chain that routes through google
     setSessionFallbackChain(sessionID, [
-      { providers: ["google"], model: "gemini-3-pro" },
+      { providers: ["google"], model: "gemini-3.1-pro-preview" },
     ])
 
     const set = setPendingModelFallback(
       sessionID,
       "Oracle",
       "google",
-      "gemini-3-pro",
+      "gemini-3.1-pro-preview",
     )
     expect(set).toBe(true)
 
     const output = {
       message: {
-        model: { providerID: "google", modelID: "gemini-3-pro" },
+        model: { providerID: "google", modelID: "gemini-3.1-pro-preview" },
       },
       parts: [{ type: "text", text: "continue" }],
     }
@@ -372,10 +372,10 @@ describe("model fallback hook", () => {
     //#when
     await hook["chat.message"]?.({ sessionID }, output)
 
-    //#then — model name should remain gemini-3-pro because no google transform exists for this ID
+    //#then: model name should remain gemini-3.1-pro-preview because no google transform exists for this ID
     expect(output.message["model"]).toEqual({
       providerID: "google",
-      modelID: "gemini-3-pro",
+      modelID: "gemini-3.1-pro-preview",
     })
 
     clearPendingModelFallback(sessionID)