core: allow agents to access global tmp directory without permission prompts

Agents can now create temporary files in the global tmp directory without
triggering external_directory permission prompts. This enables agents to
freely use temporary storage for intermediate files during builds and
other operations.

packages/core/test/global.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, test } from "bun:test"
+import fs from "fs/promises"
+import os from "os"
+import path from "path"
+import { Global } from "@opencode-ai/core/global"
+
+describe("global paths", () => {
+  test("tmp path is under the system temp directory", () => {
+    expect(Global.Path.tmp).toBe(path.join(os.tmpdir(), "opencode"))
+    expect(Global.make().tmp).toBe(Global.Path.tmp)
+  })
+
+  test("tmp path is created on module load", async () => {
+    expect((await fs.stat(Global.Path.tmp)).isDirectory()).toBe(true)
+  })
+})