feat(hooks): add auto-update-checker for plugin version management

Checks npm registry for latest version on session.created, invalidates
cache and shows toast notification when update is available.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/hooks/auto-update-checker/constants.ts

@@ -0,0 +1,40 @@
+import * as path from "node:path"
+import * as os from "node:os"
+
+export const PACKAGE_NAME = "oh-my-opencode"
+export const NPM_REGISTRY_URL = `https://registry.npmjs.org/-/package/${PACKAGE_NAME}/dist-tags`
+export const NPM_FETCH_TIMEOUT = 5000
+
+/**
+ * OpenCode plugin cache directory
+ * - Linux/macOS: ~/.cache/opencode/
+ * - Windows: %LOCALAPPDATA%/opencode/
+ */
+function getCacheDir(): string {
+  if (process.platform === "win32") {
+    return path.join(process.env.LOCALAPPDATA ?? os.homedir(), "opencode")
+  }
+  return path.join(os.homedir(), ".cache", "opencode")
+}
+
+export const CACHE_DIR = getCacheDir()
+export const VERSION_FILE = path.join(CACHE_DIR, "version")
+export const INSTALLED_PACKAGE_JSON = path.join(
+  CACHE_DIR,
+  "node_modules",
+  PACKAGE_NAME,
+  "package.json"
+)
+
+/**
+ * OpenCode config file locations (priority order)
+ */
+function getUserConfigDir(): string {
+  if (process.platform === "win32") {
+    return process.env.APPDATA ?? path.join(os.homedir(), "AppData", "Roaming")
+  }
+  return process.env.XDG_CONFIG_HOME ?? path.join(os.homedir(), ".config")
+}
+
+export const USER_CONFIG_DIR = getUserConfigDir()
+export const USER_OPENCODE_CONFIG = path.join(USER_CONFIG_DIR, "opencode", "opencode.json")