fix: Include scripts/postinstall.js in the final NPM package

[martincik]

Relates to:

No specific ticket linked - package configuration improvement

Risks

Low - Adding explicit files list to package.json for more controlled npm package contents

Background

The problem

# pnpm install
.../node_modules/@ai16z/plugin-node postinstall$ node scripts/postinstall.js
│ node:internal/modules/cjs/loader:1246
│   throw err;
│   ^
│ Error: Cannot find module '/Users/martincik/Projects/ai/access-ai-poc/echo2/node_modules/.pnpm/@[email protected]_@[email protected]_@langchain+cor…
│     at Function._resolveFilename (node:internal/modules/cjs/loader:1243:15)
│     at Function._load (node:internal/modules/cjs/loader:1068:27)
│     at TracingChannel.traceSync (node:diagnostics_channel:322:14)
│     at wrapModuleLoad (node:internal/modules/cjs/loader:219:24)
│     at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:170:5)
│     at node:internal/main/run_main_module:36:49 {
│   code: 'MODULE_NOT_FOUND',
│   requireStack: []
│ }
│ Node.js v23.2.0

Issue

The new package is triggering postintall but the scripts/postinstall.js is missing.

What does this PR do?

Adds "files" field to package.json to explicitly include scripts/postinstall.js and other essential files in the published npm package.

What kind of change is this?

Bug fixes (non-breaking change which fixes an issue)

Documentation changes needed?

My changes do not require a change to the project documentation.

Testing

Where should a reviewer start?

1. Review packages/plugin-node/package.json
2. Test npm package installation

Detailed testing steps

1. Build the package locally
2. Verify package contents include:
   • dist/
   • scripts/
   • package.json
   • LICENSE
   • tsup.config.ts
3. Test package installation

[jkbrooks]

lgtm