feat(explore): add github-copilot/gpt-5-mini to fallback chain (#1091)

* feat(explore): add github-copilot/gpt-5-mini to fallback chain

* test(explore): add tests for github-copilot/gpt-5-mini fallback

---------

Co-authored-by: Suyeol Jeon <devxoul@gmail.com>

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

@@ -712,7 +712,7 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models when
       "model": "github-copilot/claude-sonnet-4.5",
     },
     "explore": {
-      "model": "opencode/gpt-5-nano",
+      "model": "github-copilot/gpt-5-mini",
     },
     "librarian": {
       "model": "github-copilot/claude-sonnet-4.5",
@@ -776,7 +776,7 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models with
       "model": "github-copilot/claude-sonnet-4.5",
     },
     "explore": {
-      "model": "opencode/gpt-5-nano",
+      "model": "github-copilot/gpt-5-mini",
     },
     "librarian": {
       "model": "github-copilot/claude-sonnet-4.5",
@@ -1022,7 +1022,7 @@ exports[`generateModelConfig mixed provider scenarios uses OpenAI + Copilot comb
       "model": "github-copilot/claude-sonnet-4.5",
     },
     "explore": {
-      "model": "opencode/gpt-5-nano",
+      "model": "github-copilot/gpt-5-mini",
     },
     "librarian": {
       "model": "github-copilot/claude-sonnet-4.5",

src/cli/model-fallback.test.ts

@@ -353,6 +353,17 @@ describe("generateModelConfig", () => {
       // #then explore should use gpt-5-nano (fallback)
       expect(result.agents?.explore?.model).toBe("opencode/gpt-5-nano")
     })
+
+    test("explore uses gpt-5-mini when only Copilot available", () => {
+      // #given only Copilot is available
+      const config = createConfig({ hasCopilot: true })
+
+      // #when generateModelConfig is called
+      const result = generateModelConfig(config)
+
+      // #then explore should use gpt-5-mini (Copilot fallback)
+      expect(result.agents?.explore?.model).toBe("github-copilot/gpt-5-mini")
+    })
   })
 
   describe("Sisyphus agent special cases", () => {

src/cli/model-fallback.ts

@@ -139,12 +139,14 @@ export function generateModelConfig(config: InstallConfig): GeneratedOmoConfig {
       continue
     }
 
-    // Special case: explore uses Claude haiku → OpenCode gpt-5-nano
+    // Special case: explore uses Claude haiku → GitHub Copilot gpt-5-mini → OpenCode gpt-5-nano
     if (role === "explore") {
       if (avail.native.claude) {
         agents[role] = { model: "anthropic/claude-haiku-4-5" }
       } else if (avail.opencodeZen) {
         agents[role] = { model: "opencode/claude-haiku-4-5" }
+      } else if (avail.copilot) {
+        agents[role] = { model: "github-copilot/gpt-5-mini" }
       } else {
         agents[role] = { model: "opencode/gpt-5-nano" }
       }