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/script-sandbox.js

@@ -557,16 +557,23 @@ function instantiateCompiled(
  *   $workflows?: { trigger: (name: string, data?: unknown) => Promise<unknown> },
  *   pluginDir?: string | null,
  * }} [opts]
+ *
+ * When `pluginDir` is omitted, it is resolved from `script` so inspect/dry-run
+ * of `plugin/<id>` can `require()` extra packages from that plugin's node_modules.
  */
 export function instantiateScriptSource(script, source, opts = {}) {
   const compiled = compileScriptSource(source, script);
+  const pluginDir =
+    "pluginDir" in opts
+      ? opts.pluginDir ?? null
+      : (resolveScriptRef(script).pluginDir ?? null);
   const fn = instantiateCompiled(compiled, {
     log: opts.log ?? inspectLog,
     script,
     workflowName: opts.workflowName ?? "inspect",
     owner: opts.owner ?? DEFAULT_OWNER,
     $workflows: opts.$workflows,
-    pluginDir: opts.pluginDir ?? null,
+    pluginDir,
   });
   return { fn, ...extractScriptMeta(fn) };
 }