Docs: update README install instructions for officially-published marketplaces (Codex sideload path stale)

## Problem

Now that the plugin is officially published on the agent marketplaces, the **README `## Install`** section (end-user instructions) is partly out of date. In particular, the **Codex** instructions tell users to sideload the GitHub repo as a marketplace source, even though the published Codex plugin is a separately-uploaded artifact (OpenAI does not pull from this GitHub repo). Two other slugs should be verified against the live listings.

`CONTRIBUTING.md` is **not** affected — its install steps are correctly scoped to local development (`git clone`, `claude --plugin-dir`, `copilot plugin install `, `codex plugin marketplace add `), which still use local paths regardless of marketplace publication. No change needed there.

## Findings (README.md `## Install`)

| Marketplace | Current instruction | Issue |
|---|---|---|
| **Codex** (README app + CLI steps) | "Add marketplace -> Source = `https://github.com/microsoft/Dataverse-skills.git`" and `codex plugin marketplace add microsoft/Dataverse-skills` | **Stale / misleading.** This is the sideload-the-repo path. The Codex plugin is now officially published (manually uploaded to OpenAI; they do not pull from this repo), so end users should install the **official published listing**, not add the GitHub repo as a source. The repo path installs a repo-based copy that bypasses the validated/published version. |
| **Claude Code** | `/plugin install dataverse@claude-plugins-official` | **Verify** the marketplace slug `claude-plugins-official` matches the actual published Anthropic marketplace name. |
| **GitHub Copilot** | `/plugin install dataverse@awesome-copilot` | **Verify** `awesome-copilot` is the intended official distribution (community marketplace tracking this repo). |
| **Cursor** | `/add-plugin dataverse` + `cursor.com/marketplace/microsoft-dataverse` | Appears correct (official marketplace URL + listing present). Verify the URL is live. |

## What to do

1. **Codex:** replace the repo-sideload steps with the official Codex marketplace install path (browse/install the published `dataverse` listing).
2. **Claude / Copilot:** confirm the marketplace slugs (`claude-plugins-official`, `awesome-copilot`) against the live listings; fix if they differ.
3. **Cursor:** confirm the marketplace URL/listing name.
4. **CONTRIBUTING.md:** leave as-is (correctly scoped to local development).

## Acceptance

- README `## Install` reflects the **official published install path** for each marketplace (Codex especially), verified against the live listings.
- No repo-sideload instructions presented as the end-user path.
- CONTRIBUTING.md unchanged.

## Notes

Docs-only change. Whoever holds the exact official install commands per marketplace can drop them in. Surfaced while auditing install docs after the plugin's marketplace publications.

Start in README.md under the ## Install section and compare each marketplace entry with its live published listing. Replace the Codex repo-sideload instructions and correct any outdated Claude, Copilot, or Cursor details; confirm the acceptance criteria while leaving CONTRIBUTING.md unchanged.

Written by the indexing model from the issue text.