feat(hooks): add sisyphus-gpt-hephaestus-reminder hook

Shows error toast when Sisyphus runs with a GPT model, nudging user to
use Hephaestus instead.

🤖 Generated with [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/hooks/sisyphus-gpt-hephaestus-reminder/index.test.ts

@@ -0,0 +1,78 @@
+import { describe, expect, test, spyOn } from "bun:test"
+import { createSisyphusGptHephaestusReminderHook } from "./index"
+import { _resetForTesting, updateSessionAgent } from "../../features/claude-code-session-state"
+
+describe("sisyphus-gpt-hephaestus-reminder hook", () => {
+  test("shows error toast when sisyphus uses gpt model", async () => {
+    // given - sisyphus agent with gpt model
+    const showToast = spyOn({
+      fn: async () => ({}),
+    }, "fn")
+    const hook = createSisyphusGptHephaestusReminderHook({
+      client: {
+        tui: { showToast },
+      },
+    } as any)
+
+    // when - chat.message runs
+    await hook["chat.message"]?.({
+      sessionID: "ses_1",
+      agent: "sisyphus",
+      model: { providerID: "openai", modelID: "gpt-5.3-codex" },
+    })
+
+    // then - error toast is shown
+    expect(showToast).toHaveBeenCalledTimes(1)
+    expect(showToast.mock.calls[0]?.[0]).toMatchObject({
+      body: {
+        title: "Use Hephaestus for GPT Models",
+        variant: "error",
+      },
+    })
+  })
+
+  test("does not show toast for non-gpt model", async () => {
+    // given - sisyphus agent with non-gpt model
+    const showToast = spyOn({
+      fn: async () => ({}),
+    }, "fn")
+    const hook = createSisyphusGptHephaestusReminderHook({
+      client: {
+        tui: { showToast },
+      },
+    } as any)
+
+    // when - chat.message runs with claude model
+    await hook["chat.message"]?.({
+      sessionID: "ses_2",
+      agent: "sisyphus",
+      model: { providerID: "anthropic", modelID: "claude-opus-4-6" },
+    })
+
+    // then - no toast
+    expect(showToast).toHaveBeenCalledTimes(0)
+  })
+
+  test("uses session agent fallback when input agent is missing", async () => {
+    // given - session agent saved as sisyphus
+    _resetForTesting()
+    updateSessionAgent("ses_3", "sisyphus")
+    const showToast = spyOn({
+      fn: async () => ({}),
+    }, "fn")
+    const hook = createSisyphusGptHephaestusReminderHook({
+      client: {
+        tui: { showToast },
+      },
+    } as any)
+
+    // when - chat.message runs without input.agent
+    await hook["chat.message"]?.({
+      sessionID: "ses_3",
+      model: { providerID: "openai", modelID: "gpt-5.2" },
+    })
+
+    // then - toast shown via fallback agent lookup
+    expect(showToast).toHaveBeenCalledTimes(1)
+  })
+})