add colored prompt

- user@host in bold green (bold red for root), directory in bold blue
- colors are disabled with NO_COLOR, TERM=dumb or when stdout is not a terminal
- parts are sanitized before adding the color codes
- only replace the home directory with ~ on a path boundary

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main.go

@@ -303,16 +303,47 @@ func getPrompt() string {
 		hostname = "unknown"
 	}
 	
-	// Shorten the path for display
-	// Use only the last component if in home directory
+	// Replace the home directory with ~
 	home := getHomeDir()
-	if home != "" && strings.HasPrefix(wd, home) {
-		// Replace home with ~
-		displayPath := "~" + strings.TrimPrefix(wd, home)
-		return sanitize(fmt.Sprintf("%s@%s:%s $ ", username, hostname, displayPath))
+	if home != "" && (wd == home || strings.HasPrefix(wd, home+"/")) {
+		wd = "~" + strings.TrimPrefix(wd, home)
 	}
 	
-	return sanitize(fmt.Sprintf("%s@%s:%s $ ", username, hostname, wd))
+	return buildPrompt(username, hostname, wd, os.Geteuid() == 0, useColor())
+}
+
+// ANSI escape sequences used for the prompt
+const (
+	colorReset = "\x1b[0m"
+	colorUser  = "\x1b[1;32m" // bold green
+	colorRoot  = "\x1b[1;31m" // bold red
+	colorPath  = "\x1b[1;34m" // bold blue
+)
+
+// buildPrompt formats the prompt as user@host:path $. The parts are sanitized
+// before the color codes are added, so only our own escape sequences end up
+// in the prompt.
+func buildPrompt(username, hostname, path string, root, color bool) string {
+	userHost := sanitize(username + "@" + hostname)
+	path = sanitize(path)
+	if !color {
+		return userHost + ":" + path + " $ "
+	}
+	userColor := colorUser
+	if root {
+		userColor = colorRoot
+	}
+	return userColor + userHost + colorReset + ":" + colorPath + path + colorReset + " $ "
+}
+
+// useColor reports whether the prompt should be colored: stdout must be a
+// terminal, TERM must not be "dumb" and NO_COLOR (https://no-color.org) must
+// not be set.
+func useColor() bool {
+	if os.Getenv("NO_COLOR") != "" || os.Getenv("TERM") == "dumb" {
+		return false
+	}
+	return readline.IsTerminal(int(os.Stdout.Fd()))
 }
 
 // shellCompleter implements readline.AutoCompleter for contextual completion
@@ -666,6 +697,8 @@ Features:
     override with $GOSH_HISTFILE); lines starting with a space are not saved
   - Tab completion for commands and file paths
   - Tilde expansion (~, ~/path) at the start of unquoted words
+  - Colored prompt (user@host green, red for root; directory blue),
+    disabled with $NO_COLOR, TERM=dumb or when output is not a terminal
   - Environment variable expansion ($VAR, ${VAR}, ${VAR:-default});
     using an unset variable is an error, use ${VAR:-} for an empty default`)
 		return nil