Add GITHUB_TOKEN fallback in GitHub connector authentication

Co-authored-by: Malcolmnixon <1863707+Malcolmnixon@users.noreply.github.com>

Author: Copilot

THEORY-OF-OPERATIONS.md

@@ -82,8 +82,9 @@ BuildMark also runs `git rev-parse --abbrev-ref HEAD` to determine the current b
 ##### 2. Authenticate with GitHub
 
 BuildMark reads an authentication token from environment variables. It checks `GH_TOKEN` first,
-then falls back to `GITHUB_TOKEN`. If no token is found, unauthenticated requests are made, which
-are subject to lower API rate limits (60 requests per hour versus 5,000 with a token).
+then falls back to `GITHUB_TOKEN`. If neither environment variable is set, BuildMark attempts to
+obtain a token by running `gh auth token` (the GitHub CLI). If no token can be obtained from any
+of these sources, BuildMark fails with an error.
 
 ##### 3. Fetch repository data
 

src/DemaConsulting.BuildMark/RepoConnectors/GitHubRepoConnector.cs

@@ -875,21 +875,28 @@ private static string GetTypeFromLabels(IReadOnlyList<PullRequestLabelInfo> labe
     /// <returns>GitHub token.</returns>
     private async Task<string> GetGitHubTokenAsync()
     {
-        // Try to get token from environment variable first
+        // Try GH_TOKEN environment variable first
         var token = Environment.GetEnvironmentVariable("GH_TOKEN");
         if (!string.IsNullOrEmpty(token))
         {
             return token;
         }
 
+        // Try GITHUB_TOKEN environment variable next
+        token = Environment.GetEnvironmentVariable("GITHUB_TOKEN");
+        if (!string.IsNullOrEmpty(token))
+        {
+            return token;
+        }
+
         // Fall back to gh auth token
         try
         {
             return await RunCommandAsync("gh", "auth token");
         }
         catch (InvalidOperationException)
         {
-            throw new InvalidOperationException("Unable to get GitHub token. Set GH_TOKEN environment variable or authenticate with 'gh auth login'.");
+            throw new InvalidOperationException("Unable to get GitHub token. Set GH_TOKEN or GITHUB_TOKEN environment variable or authenticate with 'gh auth login'.");
         }
     }