Add GitHub CLI installation guide and link from README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rtfisher

HOWTO_install_gh.md

@@ -0,0 +1,46 @@
+# How to Install the GitHub CLI (`gh`)
+
+The GitHub CLI is required for creating and connecting to GitHub Codespaces from the command line.
+
+## macOS
+
+```bash
+brew install gh
+```
+
+## Linux (Debian/Ubuntu)
+
+```bash
+sudo apt update
+sudo apt install gh
+```
+
+## Linux (Fedora/RHEL)
+
+```bash
+sudo dnf install gh
+```
+
+## Windows
+
+Open **PowerShell** (press **Win + R**, type `powershell`, press Enter) and run:
+
+```
+winget install --id GitHub.cli
+```
+
+## After Installation
+
+Authenticate with your GitHub account:
+
+```bash
+gh auth login
+```
+
+Add the Codespaces scope:
+
+```bash
+gh auth refresh -h github.com -s codespace
+```
+
+You are now ready to create and connect to Codespaces.

README.md

@@ -56,7 +56,7 @@ cd ~/flashx/Flash-X/object
 ./flashx
 ```
 
-**From the command line (GitHub CLI):**
+**From the command line ([GitHub CLI](HOWTO_install_gh.md)):**
 
 ```bash
 # Create a codespace (prints the codespace name on success)