Add support for GITHUB_ACCESS_TOKEN env var

- Support GITHUB_ACCESS_TOKEN as the primary environment variable
- Keep GITHUB_PERSONAL_ACCESS_TOKEN as deprecated fallback
- GITHUB_ACCESS_TOKEN takes precedence when both are set
- Update error message to mention both variables
- All existing tests pass

Co-authored-by: SamMorrowDrums <[email protected]>

Author: Copilot

cmd/github-mcp-server/main.go

@@ -32,9 +32,14 @@ var (
 		Short: "Start stdio server",
 		Long:  `Start a server that communicates via standard input/output streams using JSON-RPC messages.`,
 		RunE: func(_ *cobra.Command, _ []string) error {
-			token := viper.GetString("personal_access_token")
+			// Check for GITHUB_ACCESS_TOKEN first (preferred), then fall back to
+			// GITHUB_PERSONAL_ACCESS_TOKEN (deprecated but still supported)
+			token := viper.GetString("access_token")
 			if token == "" {
-				return errors.New("GITHUB_PERSONAL_ACCESS_TOKEN not set")
+				token = viper.GetString("personal_access_token")
+			}
+			if token == "" {
+				return errors.New("GITHUB_ACCESS_TOKEN or GITHUB_PERSONAL_ACCESS_TOKEN not set")
 			}
 
 			// If you're wondering why we're not using viper.GetStringSlice("toolsets"),