fix(cli): add -preview suffix for GitHub Copilot Gemini model names

GitHub Copilot uses gemini-3-pro-preview and gemini-3-flash-preview as
the official model identifiers. The CLI installer was generating config
with incorrect names (gemini-3-pro, gemini-3-flash).

Reported by user: the install command was creating config with wrong
model names that don't work with GitHub Copilot API.

src/cli/__snapshots__/model-fallback.test.ts.snap

@@ -760,7 +760,7 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models when
       "variant": "medium",
     },
     "multimodal-looker": {
-      "model": "github-copilot/gemini-3-flash",
+      "model": "github-copilot/gemini-3-flash-preview",
     },
     "oracle": {
       "model": "github-copilot/gpt-5.2",
@@ -777,7 +777,7 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models when
   },
   "categories": {
     "artistry": {
-      "model": "github-copilot/gemini-3-pro",
+      "model": "github-copilot/gemini-3-pro-preview",
       "variant": "max",
     },
     "deep": {
@@ -798,10 +798,10 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models when
       "model": "github-copilot/claude-sonnet-4.5",
     },
     "visual-engineering": {
-      "model": "github-copilot/gemini-3-pro",
+      "model": "github-copilot/gemini-3-pro-preview",
     },
     "writing": {
-      "model": "github-copilot/gemini-3-flash",
+      "model": "github-copilot/gemini-3-flash-preview",
     },
   },
 }
@@ -833,7 +833,7 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models with
       "variant": "medium",
     },
     "multimodal-looker": {
-      "model": "github-copilot/gemini-3-flash",
+      "model": "github-copilot/gemini-3-flash-preview",
     },
     "oracle": {
       "model": "github-copilot/gpt-5.2",
@@ -850,7 +850,7 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models with
   },
   "categories": {
     "artistry": {
-      "model": "github-copilot/gemini-3-pro",
+      "model": "github-copilot/gemini-3-pro-preview",
       "variant": "max",
     },
     "deep": {
@@ -872,10 +872,10 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models with
       "model": "github-copilot/claude-sonnet-4.5",
     },
     "visual-engineering": {
-      "model": "github-copilot/gemini-3-pro",
+      "model": "github-copilot/gemini-3-pro-preview",
     },
     "writing": {
-      "model": "github-copilot/gemini-3-flash",
+      "model": "github-copilot/gemini-3-flash-preview",
     },
   },
 }
@@ -1090,7 +1090,7 @@ exports[`generateModelConfig mixed provider scenarios uses OpenAI + Copilot comb
       "variant": "medium",
     },
     "multimodal-looker": {
-      "model": "github-copilot/gemini-3-flash",
+      "model": "github-copilot/gemini-3-flash-preview",
     },
     "oracle": {
       "model": "openai/gpt-5.2",
@@ -1107,7 +1107,7 @@ exports[`generateModelConfig mixed provider scenarios uses OpenAI + Copilot comb
   },
   "categories": {
     "artistry": {
-      "model": "github-copilot/gemini-3-pro",
+      "model": "github-copilot/gemini-3-pro-preview",
       "variant": "max",
     },
     "deep": {
@@ -1128,10 +1128,10 @@ exports[`generateModelConfig mixed provider scenarios uses OpenAI + Copilot comb
       "model": "github-copilot/claude-sonnet-4.5",
     },
     "visual-engineering": {
-      "model": "github-copilot/gemini-3-pro",
+      "model": "github-copilot/gemini-3-pro-preview",
     },
     "writing": {
-      "model": "github-copilot/gemini-3-flash",
+      "model": "github-copilot/gemini-3-flash-preview",
     },
   },
 }
@@ -1290,7 +1290,7 @@ exports[`generateModelConfig mixed provider scenarios uses all fallback provider
       "variant": "medium",
     },
     "multimodal-looker": {
-      "model": "github-copilot/gemini-3-flash",
+      "model": "github-copilot/gemini-3-flash-preview",
     },
     "oracle": {
       "model": "github-copilot/gpt-5.2",
@@ -1307,7 +1307,7 @@ exports[`generateModelConfig mixed provider scenarios uses all fallback provider
   },
   "categories": {
     "artistry": {
-      "model": "github-copilot/gemini-3-pro",
+      "model": "github-copilot/gemini-3-pro-preview",
       "variant": "max",
     },
     "deep": {
@@ -1328,10 +1328,10 @@ exports[`generateModelConfig mixed provider scenarios uses all fallback provider
       "model": "github-copilot/claude-sonnet-4.5",
     },
     "visual-engineering": {
-      "model": "github-copilot/gemini-3-pro",
+      "model": "github-copilot/gemini-3-pro-preview",
     },
     "writing": {
-      "model": "github-copilot/gemini-3-flash",
+      "model": "github-copilot/gemini-3-flash-preview",
     },
   },
 }

src/cli/model-fallback.ts

@@ -75,6 +75,8 @@ function transformModelForProvider(provider: string, model: string): string {
       .replace("claude-sonnet-4-5", "claude-sonnet-4.5")
       .replace("claude-haiku-4-5", "claude-haiku-4.5")
       .replace("claude-sonnet-4", "claude-sonnet-4")
+      .replace("gemini-3-pro", "gemini-3-pro-preview")
+      .replace("gemini-3-flash", "gemini-3-flash-preview")
   }
   return model
 }