fix(opencode): use AGENTS.md as prompt filename

OpenCode follows the same convention as Codex, using AGENTS.md
instead of OPENCODE.md for the system prompt file.

src/components/prompts/PromptFormModal.tsx

@@ -34,7 +34,7 @@ const PromptFormModal: React.FC<PromptFormModalProps> = ({
     claude: "CLAUDE.md",
     codex: "AGENTS.md",
     gemini: "GEMINI.md",
-    opencode: "OPENCODE.md",
+    opencode: "AGENTS.md",
   };
   const filename = filenameMap[appId];
   const [name, setName] = useState("");

src/components/prompts/PromptFormPanel.tsx

@@ -28,7 +28,7 @@ const PromptFormPanel: React.FC<PromptFormPanelProps> = ({
     claude: "CLAUDE.md",
     codex: "AGENTS.md",
     gemini: "GEMINI.md",
-    opencode: "OPENCODE.md",
+    opencode: "AGENTS.md",
   };
   const filename = filenameMap[appId];
   const [name, setName] = useState("");