Derive tmux runtime fallback from schema defaults

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/create-runtime-tmux-config.ts

@@ -0,0 +1,6 @@
+import type { OhMyOpenCodeConfig, TmuxConfig } from "./config"
+import { TmuxConfigSchema } from "./config/schema/tmux"
+
+export function createRuntimeTmuxConfig(pluginConfig: { tmux?: OhMyOpenCodeConfig["tmux"] }): TmuxConfig {
+  return TmuxConfigSchema.parse(pluginConfig.tmux ?? {})
+}