docs(readme): Add Cursor install instructions once published to Cursor Marketplace

## Summary

Add a **Cursor** subsection under `## Install` in [`README.md`](https://github.com/microsoft/Dataverse-skills/blob/main/README.md) once the plugin is published to the Cursor Marketplace.

## Background

PR #68 ([feat: Add Cursor plugin support](https://github.com/microsoft/Dataverse-skills/pull/68)) introduces full Cursor support in the plugin (manifests, `dv-connect` Cursor branch, `mcp-configuration.md` Cursor branch). It originally included a README section as well, but that section was reverted before merge because it instructs users to *"Install from the Cursor Marketplace (search for Microsoft Dataverse)"* — wording that is only accurate once the plugin is actually listed on the marketplace.

## Action

After the plugin is approved and listed at :

1. Add a `### Cursor` subsection to `README.md` under `## Install`, mirroring the existing **GitHub Copilot** and **Claude Code** sections.
2. Verify the install command / search phrase against the published marketplace listing.
3. Include a tip pointing at `dataverse auth create --environment ` as the recommended first command (so the MCP stdio proxy and `scripts/auth.py` reuse the same MSAL cache silently — no device-code prompt).

## Draft copy (from the reverted PR commit, ready to drop in after publishing)

```markdown
### Cursor

Install from the [Cursor Marketplace](https://cursor.com/marketplace) (search for "Microsoft Dataverse").

After installation, ask your agent:

> "Connect to Dataverse"

The `dv-connect` skill walks through tool installation, authentication, and registers a `dataverse-` MCP server entry in `~/.cursor/mcp.json` (or `/.cursor/mcp.json` for project scope). Reload your Cursor window to activate the MCP server.

> **Tip:** run `dataverse auth create --environment ` once before the first MCP call — the MCP stdio proxy and `scripts/auth.py` both reuse that cache silently, so no device-code prompt.
```

## Related

- PR #68 — Cursor plugin support (the work this issue follows up on)

Once the plugin is listed, open README.md and review the existing GitHub Copilot and Claude Code sections under ## Install. Verify the Cursor Marketplace install wording and search phrase against the published listing, then add the provided Cursor subsection and confirm it includes the recommended dataverse auth create --environment tip.

Written by the indexing model from the issue text.