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/oauth_test.go

@@ -0,0 +1,199 @@
+package auth
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestBuildAuthorizeURL(t *testing.T) {
+	cfg := OAuthProviderConfig{
+		Issuer:   "https://auth.example.com",
+		ClientID: "test-client-id",
+		Scopes:   "openid profile",
+		Port:     1455,
+	}
+	pkce := PKCECodes{
+		CodeVerifier:  "test-verifier",
+		CodeChallenge: "test-challenge",
+	}
+
+	u := BuildAuthorizeURL(cfg, pkce, "test-state", "http://localhost:1455/auth/callback")
+
+	if !strings.HasPrefix(u, "https://auth.example.com/authorize?") {
+		t.Errorf("URL does not start with expected prefix: %s", u)
+	}
+	if !strings.Contains(u, "client_id=test-client-id") {
+		t.Error("URL missing client_id")
+	}
+	if !strings.Contains(u, "code_challenge=test-challenge") {
+		t.Error("URL missing code_challenge")
+	}
+	if !strings.Contains(u, "code_challenge_method=S256") {
+		t.Error("URL missing code_challenge_method")
+	}
+	if !strings.Contains(u, "state=test-state") {
+		t.Error("URL missing state")
+	}
+	if !strings.Contains(u, "response_type=code") {
+		t.Error("URL missing response_type")
+	}
+}
+
+func TestParseTokenResponse(t *testing.T) {
+	resp := map[string]interface{}{
+		"access_token":  "test-access-token",
+		"refresh_token": "test-refresh-token",
+		"expires_in":    3600,
+		"id_token":      "test-id-token",
+	}
+	body, _ := json.Marshal(resp)
+
+	cred, err := parseTokenResponse(body, "openai")
+	if err != nil {
+		t.Fatalf("parseTokenResponse() error: %v", err)
+	}
+
+	if cred.AccessToken != "test-access-token" {
+		t.Errorf("AccessToken = %q, want %q", cred.AccessToken, "test-access-token")
+	}
+	if cred.RefreshToken != "test-refresh-token" {
+		t.Errorf("RefreshToken = %q, want %q", cred.RefreshToken, "test-refresh-token")
+	}
+	if cred.Provider != "openai" {
+		t.Errorf("Provider = %q, want %q", cred.Provider, "openai")
+	}
+	if cred.AuthMethod != "oauth" {
+		t.Errorf("AuthMethod = %q, want %q", cred.AuthMethod, "oauth")
+	}
+	if cred.ExpiresAt.IsZero() {
+		t.Error("ExpiresAt should not be zero")
+	}
+}
+
+func TestParseTokenResponseNoAccessToken(t *testing.T) {
+	body := []byte(`{"refresh_token": "test"}`)
+	_, err := parseTokenResponse(body, "openai")
+	if err == nil {
+		t.Error("expected error for missing access_token")
+	}
+}
+
+func TestExchangeCodeForTokens(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/oauth/token" {
+			http.Error(w, "not found", http.StatusNotFound)
+			return
+		}
+		if r.Method != http.MethodPost {
+			http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+			return
+		}
+
+		r.ParseForm()
+		if r.FormValue("grant_type") != "authorization_code" {
+			http.Error(w, "invalid grant_type", http.StatusBadRequest)
+			return
+		}
+
+		resp := map[string]interface{}{
+			"access_token":  "mock-access-token",
+			"refresh_token": "mock-refresh-token",
+			"expires_in":    3600,
+		}
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	cfg := OAuthProviderConfig{
+		Issuer:   server.URL,
+		ClientID: "test-client",
+		Scopes:   "openid",
+		Port:     1455,
+	}
+
+	cred, err := exchangeCodeForTokens(cfg, "test-code", "test-verifier", "http://localhost:1455/auth/callback")
+	if err != nil {
+		t.Fatalf("exchangeCodeForTokens() error: %v", err)
+	}
+
+	if cred.AccessToken != "mock-access-token" {
+		t.Errorf("AccessToken = %q, want %q", cred.AccessToken, "mock-access-token")
+	}
+}
+
+func TestRefreshAccessToken(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/oauth/token" {
+			http.Error(w, "not found", http.StatusNotFound)
+			return
+		}
+
+		r.ParseForm()
+		if r.FormValue("grant_type") != "refresh_token" {
+			http.Error(w, "invalid grant_type", http.StatusBadRequest)
+			return
+		}
+
+		resp := map[string]interface{}{
+			"access_token":  "refreshed-access-token",
+			"refresh_token": "refreshed-refresh-token",
+			"expires_in":    3600,
+		}
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	cfg := OAuthProviderConfig{
+		Issuer:   server.URL,
+		ClientID: "test-client",
+	}
+
+	cred := &AuthCredential{
+		AccessToken:  "old-token",
+		RefreshToken: "old-refresh-token",
+		Provider:     "openai",
+		AuthMethod:   "oauth",
+	}
+
+	refreshed, err := RefreshAccessToken(cred, cfg)
+	if err != nil {
+		t.Fatalf("RefreshAccessToken() error: %v", err)
+	}
+
+	if refreshed.AccessToken != "refreshed-access-token" {
+		t.Errorf("AccessToken = %q, want %q", refreshed.AccessToken, "refreshed-access-token")
+	}
+	if refreshed.RefreshToken != "refreshed-refresh-token" {
+		t.Errorf("RefreshToken = %q, want %q", refreshed.RefreshToken, "refreshed-refresh-token")
+	}
+}
+
+func TestRefreshAccessTokenNoRefreshToken(t *testing.T) {
+	cfg := OpenAIOAuthConfig()
+	cred := &AuthCredential{
+		AccessToken: "old-token",
+		Provider:    "openai",
+		AuthMethod:  "oauth",
+	}
+
+	_, err := RefreshAccessToken(cred, cfg)
+	if err == nil {
+		t.Error("expected error for missing refresh token")
+	}
+}
+
+func TestOpenAIOAuthConfig(t *testing.T) {
+	cfg := OpenAIOAuthConfig()
+	if cfg.Issuer != "https://auth.openai.com" {
+		t.Errorf("Issuer = %q, want %q", cfg.Issuer, "https://auth.openai.com")
+	}
+	if cfg.ClientID == "" {
+		t.Error("ClientID is empty")
+	}
+	if cfg.Port != 1455 {
+		t.Errorf("Port = %d, want 1455", cfg.Port)
+	}
+}