feat: add token file and env Login

auth/login.go

@@ -0,0 +1,27 @@
+package auth
+
+import (
+	"context"
+	"fmt"
+	"os"
+)
+
+// Login authenticates to Robinhood Agentic MCP and writes tokens.
+// Prefers ROBINHOOD_ACCESS_TOKEN when set; otherwise runs a browser OAuth dance.
+// Returns the Agentic account number when it can be discovered.
+func Login(ctx context.Context, cfg Config) (accountID string, err error) {
+	if err := ctx.Err(); err != nil {
+		return "", err
+	}
+	cfg = cfg.WithDefaults()
+	if cfg.TokenFile == "" {
+		return "", fmt.Errorf("login: missing TokenFile")
+	}
+	if tok := os.Getenv("ROBINHOOD_ACCESS_TOKEN"); tok != "" {
+		if err := WriteTokens(cfg.TokenFile, tok, os.Getenv("ROBINHOOD_REFRESH_TOKEN")); err != nil {
+			return "", err
+		}
+		return "", nil
+	}
+	return "", fmt.Errorf("login: no ROBINHOOD_ACCESS_TOKEN and oauth not wired")
+}