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/hook.ts

@@ -0,0 +1,37 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import { getSessionAgent } from "../../features/claude-code-session-state"
+import { log } from "../../shared"
+
+const TOAST_TITLE = "Use Hephaestus for GPT Models"
+const TOAST_MESSAGE = "Sisyphus is using a GPT model. Use Hephaestus and include 'ulw' in your prompt."
+
+export function createSisyphusGptHephaestusReminderHook(ctx: PluginInput) {
+  return {
+    "chat.message": async (input: {
+      sessionID: string
+      agent?: string
+      model?: { providerID: string; modelID: string }
+    }): Promise<void> => {
+      const agentName = (input.agent ?? getSessionAgent(input.sessionID) ?? "").toLowerCase()
+      const modelID = input.model?.modelID?.toLowerCase() ?? ""
+
+      if (agentName !== "sisyphus" || !modelID.includes("gpt")) {
+        return
+      }
+
+      await ctx.client.tui.showToast({
+        body: {
+          title: TOAST_TITLE,
+          message: TOAST_MESSAGE,
+          variant: "error",
+          duration: 5000,
+        },
+      }).catch((error) => {
+        log("[sisyphus-gpt-hephaestus-reminder] Failed to show toast", {
+          sessionID: input.sessionID,
+          error,
+        })
+      })
+    },
+  }
+}