Proposal: Add Google Drive Folder support to gatekeeper-google

Hi everyone,

First of all, congratulations on Cloudflare OS v2! The architecture built on top of Durable Objects, Dynamic Workers, and Cap'n Web RPC is incredibly elegant and a joy to study.

### The Problem / Motivation
Currently, the `gatekeeper-google` worker is highly focused on granular single-document access (e.g. `GOOGLE_DOC_RESOURCE`). While this is perfect for strict least-privilege security, it introduces a friction point for teams that manage dynamic sets of files.

For instance, in our company, meeting transcriptions, marketing copies, and team logs are continually added to a shared Google Drive folder. Requiring users (or agents) to manually search and connect every new document individually is tedious and breaks automation flows.

### Proposed Solution
To bridge this gap while preserving the system's strong capability-based security, I have implemented and locally verified a **Google Drive Folder** resource. It allows teams to connect a specific folder (via `https://drive.google.com/drive/folders/:folderId`) and gives agents read-only access to files inside it.

I designed the API in `drive-folder-types.d.ts` following the exact conventions of the repository:

```typescript
export type DriveFolderMetadata = {
title: string;
};

export type DriveFileSummary = {
id: string;
name: string;
mimeType: string;
lastModified: Date;
};

export interface GoogleDriveFolderSession {
getMetadata(): Promise;
listFiles(): Promise;
getFileContent(fileId: string): Promise;
}
```

### Technical Highlights of our Implementation:
1. **Strict Scope Verification (No Leakage):** In the session implementation (`getFileContent`), we fetch the file's metadata first and explicitly verify that the file's parents array includes the connected `folderId`. This prevents any malicious prompt-injection or agent bypass from reading documents outside the configured folder.
2. **Automatic Markdown Conversion:** If a file inside the folder is a native Google Doc (`application/vnd.google-apps.document`), it automatically routes through the existing `GoogleDocsApi` and `docToMarkdown` converter, delivering clean Markdown. For plain text files, it falls back to a raw `alt=media` fetch.
3. **Sharing Security (Strategy B Observer):** We implemented `hasDriveFolderAccess` in `GoogleVerifier` and `GoogleVerifierApi`, ensuring that when a Gadget is shared, collaborators are properly checked for access against the Google Drive API before observing the folder data.
4. **Clean Monorepo Integration:** It includes a TSX-based configurator UI (`google-drive-folder-configurator-ui.tsx` via `@gadgets/configurator-ui`) and is registered under Durable Object migration `v4`.

The entire implementation compiles and type-checks successfully across the workspace with zero errors (`pnpm lint` and `pnpm types:check` pass cleanly).

### Next Steps
Since we understand the repository holds a very high bar for code changes and PR reviews, I wanted to submit this idea first.

If this feature aligns with your roadmap, I would be more than happy to open a clean, beautifully formatted Pull Request with these changes. Let me know your thoughts!

Best regards,
Kelvin Rosa