Add SETUP with verified hub auth steps

Author: sorochak

SETUP.md

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+# Setup — read before the tutorial
+
+Everything you type in this session runs on CryoCloud. This page gets you
+logged in and able to push to GitHub from the hub. If any step fails during the
+session, let us know and a facilitator will help get you unstuck.
+
+## Before you arrive
+
+1. Have a GitHub account.
+2. Accept the invitation to the **OceanHackWeek GitHub organization** and to the
+   `ohw26-participants` team (check the email you used to sign up for GitHub).
+3. **Make your org membership Public.** CryoCloud reads your team membership to
+   log you in, and GitHub defaults membership to *Private*. Go to
+   `https://github.com/orgs/oceanhackweek/people/<your-handle>` and confirm you
+   are **not** set to private. If you're private, login to the hub will fail.
+
+## Logging in to CryoCloud
+
+1. Go to `https://hub.cryointhecloud.com` and sign in with GitHub. The first
+   time, authorize **nasa-cryo-prod** (2i2c) when asked — you should see
+   `oceanhackweek` with a checkmark on that screen.
+2. On the Server Options screen:
+   * Environment: choose the OceanHackWeek Python Image.
+   * Resource Allocation: ~7 GB RAM is a good general choice. (This tutorial barely 
+     uses any memory, so a smaller size works too, but you'll use this same server 
+     for the data tutorials later in the week.)
+   * Click Start.
+3. Wait through the startup spinner (a new node can take a few minutes to come
+   up, especially when everyone logs in at once), and you'll land in JupyterLab.
+4. Open a Terminal: in the Launcher, under Other, Terminal.
+
+## One-time git setup (in the hub terminal)
+
+```bash
+git config --global user.name "Your Name"
+git config --global user.email "the-email-you-used-for-github@example.com"
+git config --global core.pager cat      # the hub has no 'less'; this stops git log/config erroring
+git config --global pull.rebase false
+```
+
+## Authenticating git
+
+The hub can't reuse your laptop's credentials, so you authenticate here with GitHub's CLI. Run:
+
+```bash
+gh auth login
+```
+
+Answer the prompts with the arrow keys:
+
+1. **Where do you use GitHub?** → `GitHub.com`
+2. **Preferred protocol for Git operations?** → `HTTPS`
+3. **Authenticate Git with your GitHub credentials?** → `Yes`
+4. **How would you like to authenticate?** → `Login with a web browser`
+
+It shows a **one-time code** (like `XXXX-XXXX`). Copy it, then on **your own
+laptop's browser** open `https://github.com/login/device`, paste the code, and
+authorize. (Authorizing lists `oceanhackweek` — that's expected.)
+
+> **Heads up — you'll see a red message like**
+> `Failed opening a web browser at https://github.com/login/device ... executable
+> file not found`. **That is normal.** The hub simply has no browser to open for
+> you, which is why you open the URL on your own laptop instead. Authentication
+> still completes — look for `✓ Authentication complete` and
+> `✓ Logged in as <your-handle>`.
+
+That's it, git can now push from the hub. (Your credentials are stored on this
+server, another reason to shut the server down when you're done.)
+
+## Confirm you're authenticated
+
+Check that git is ready to talk to GitHub:
+
+```bash
+gh auth status
+```
+
+You should see something like `✓ Logged in to github.com account <your-handle>`.
+If you do, you're set. We'll clone the activity repo together as the first step
+of the tutorial.
+
+## When you're done for the day
+
+Shut your server down so it isn't billing idle compute: **File → Hub Control
+Panel → Stop My Server** (or go to `https://hub.cryointhecloud.com/hub/home`).
+Stopping the server does **not** delete anything in your home directory.