core: move Global module to @opencode-ai/core for centralized path management

Move the Global module from packages/opencode/src/global to packages/core/src/global
to provide a unified location for managing XDG directories and application paths.
This eliminates duplicate path definitions across packages and ensures consistent
access to data, config, cache, state, log, and bin directories throughout the codebase.

packages/opencode/src/util/which.ts

@@ -1,6 +1,6 @@
 import whichPkg from "which"
 import path from "path"
-import { Global } from "../global"
+import { Global } from "@opencode-ai/core/global"
 
 export function which(cmd: string, env?: NodeJS.ProcessEnv) {
   const base = env?.PATH ?? env?.Path ?? process.env.PATH ?? process.env.Path ?? ""