Add upgrade command to CLI

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@because/sandlot",
-  "version": "0.0.19",
+  "version": "0.0.20",
   "description": "Sandboxed, branch-based development with Claude",
   "type": "module",
   "bin": {

src/cli.ts

@@ -183,6 +183,18 @@ program
 
 registerVmCommands(program)
 
+program
+  .command("upgrade")
+  .description("Upgrade sandlot to the latest version")
+  .action(async () => {
+    const result = await Bun.spawn(["bun", "install", "-g", "@because/sandlot@latest"], {
+      stdin: "inherit",
+      stdout: "inherit",
+      stderr: "inherit",
+    }).exited
+    process.exit(result)
+  })
+
 program
   .command("version")
   .description("Print the version number")