feat(auth): add OAuth and token-based login for OpenAI and Anthropic

Add `picoclaw auth` CLI command supporting:
- OpenAI OAuth2 (PKCE + browser callback or device code flow)
- Anthropic paste-token flow
- Token storage at ~/.picoclaw/auth.json with 0600 permissions
- Auto-refresh for expired OAuth tokens in provider

Closes #18

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/auth/pkce_test.go

@@ -0,0 +1,51 @@
+package auth
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"testing"
+)
+
+func TestGeneratePKCE(t *testing.T) {
+	codes, err := GeneratePKCE()
+	if err != nil {
+		t.Fatalf("GeneratePKCE() error: %v", err)
+	}
+
+	if codes.CodeVerifier == "" {
+		t.Fatal("CodeVerifier is empty")
+	}
+	if codes.CodeChallenge == "" {
+		t.Fatal("CodeChallenge is empty")
+	}
+
+	verifierBytes, err := base64.RawURLEncoding.DecodeString(codes.CodeVerifier)
+	if err != nil {
+		t.Fatalf("CodeVerifier is not valid base64url: %v", err)
+	}
+	if len(verifierBytes) != 64 {
+		t.Errorf("CodeVerifier decoded length = %d, want 64", len(verifierBytes))
+	}
+
+	hash := sha256.Sum256([]byte(codes.CodeVerifier))
+	expectedChallenge := base64.RawURLEncoding.EncodeToString(hash[:])
+	if codes.CodeChallenge != expectedChallenge {
+		t.Errorf("CodeChallenge = %q, want SHA256 of verifier = %q", codes.CodeChallenge, expectedChallenge)
+	}
+}
+
+func TestGeneratePKCEUniqueness(t *testing.T) {
+	codes1, err := GeneratePKCE()
+	if err != nil {
+		t.Fatalf("GeneratePKCE() error: %v", err)
+	}
+
+	codes2, err := GeneratePKCE()
+	if err != nil {
+		t.Fatalf("GeneratePKCE() error: %v", err)
+	}
+
+	if codes1.CodeVerifier == codes2.CodeVerifier {
+		t.Error("two GeneratePKCE() calls produced identical verifiers")
+	}
+}