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.test.ts

@@ -0,0 +1,17 @@
+/// <reference types="bun-types" />
+
+import { describe, expect, test } from "bun:test"
+
+import { TmuxConfigSchema } from "./config/schema/tmux"
+import { createRuntimeTmuxConfig } from "./create-runtime-tmux-config"
+
+describe("createRuntimeTmuxConfig", () => {
+  describe("#given tmux isolation is omitted from plugin config", () => {
+    test("#when runtime tmux config is created #then it matches the schema default", () => {
+      const runtimeTmuxConfig = createRuntimeTmuxConfig({})
+      const schemaDefault = TmuxConfigSchema.parse({}).isolation
+
+      expect(runtimeTmuxConfig.isolation).toBe(schemaDefault)
+    })
+  })
+})