fix(plugins): enhance plugin installation and script handling

- Updated the `pnpm install` command in `plugin-install.js` to include the `--ignore-workspace` flag, ensuring proper installation of plugins within the app tree.
- Improved the `instantiateScriptSource` function in `script-sandbox.js` to resolve `pluginDir` more effectively, allowing for better package management.
- Added a new smoke test in `plugins-smoke.js` to validate the ability to require additional packages from plugin directories, enhancing testing coverage for plugin functionality.
- Updated documentation in `AGENTS.md` to reflect changes in the installation command.

packages/server/plugin-install.js

@@ -51,9 +51,18 @@ export async function pnpmInstallPlugin(dir) {
   }
   if (!hasDeps) return { skipped: true };
 
+  // --ignore-workspace: plugins live under the app tree but are not workspace
+  // packages; without this, pnpm install --dir can no-op against the root monorepo.
   await execFileAsync(
     "pnpm",
-    ["install", "--dir", dir, "--ignore-scripts", "--prefer-offline"],
+    [
+      "install",
+      "--dir",
+      dir,
+      "--ignore-scripts",
+      "--prefer-offline",
+      "--ignore-workspace",
+    ],
     {
       cwd: dir,
       env: { ...process.env, CI: "1" },