docs: add git + GitHub CLI install and auth instructions

Covers Debian/Ubuntu apt install for git, adding GitHub's official
apt repository (Linux tap equivalent) to install gh CLI, gh auth login
flow, and using gh repo clone as the step-0 clone command so credentials
are handled automatically.

https://claude.ai/code/session_012eTokAaGiZo7aGt1T2W9BC

README.md

@@ -113,13 +113,57 @@ Look for any of these in the app's docs:
 | Vaultwarden | No | Use Authelia OIDC integration instead |
 | Router/NAS admin | Rarely | Use case 3 (2FA gate) or case 4 |
 
+## Getting git and authenticating to GitHub
+
+If git isn't installed on the server yet:
+
+```bash
+# Debian / Ubuntu / Raspberry Pi OS
+sudo apt update && sudo apt install -y git
+
+# Fedora / RHEL / Rocky / AlmaLinux
+sudo dnf install -y git
+```
+
+The easiest way to authenticate is the **GitHub CLI** (`gh`). Install it by
+adding GitHub's official apt repository (their Linux equivalent of a Homebrew
+tap), then run `gh auth login` to authenticate interactively:
+
+```bash
+# Add the GitHub CLI apt repository
+sudo apt install -y curl
+curl -fsSL https://cli.github.com/packages/githubcli-archive-keyring.gpg \
+    | sudo dd of=/usr/share/keyrings/githubcli-archive-keyring.gpg
+sudo chmod go+r /usr/share/keyrings/githubcli-archive-keyring.gpg
+echo "deb [arch=$(dpkg --print-architecture) \
+    signed-by=/usr/share/keyrings/githubcli-archive-keyring.gpg] \
+    https://cli.github.com/packages stable main" \
+    | sudo tee /etc/apt/sources.list.d/github-cli.list > /dev/null
+sudo apt update && sudo apt install -y gh
+
+# Authenticate -- follow the prompts (browser or paste a token)
+gh auth login
+```
+
+When prompted: choose **GitHub.com**, **HTTPS**, and **Login with a web
+browser** (or paste a personal access token if the server has no browser).
+Once done, `gh` passes credentials to `git` automatically -- no extra config
+needed.
+
+On macOS the whole thing is two lines:
+
+```bash
+brew install git gh
+gh auth login
+```
+
 ## First-run setup
 
 ```bash
-# 0) Clone the repo onto the server (auth stack -- separate from the
-#    Frigate stack which lives on the `main` branch).
-git clone -b authelia https://github.com/outis1one/frigate_w_audio.git \
-    ~/docker/authelia
+# 0) Clone the auth stack onto the server.
+#    (The Frigate stack lives on the `main` branch and is cloned separately.)
+gh repo clone outis1one/frigate_w_audio -- \
+    --branch authelia ~/docker/authelia
 cd ~/docker/authelia
 
 # 1) Create the external docker network (Caddy must also be on this).