feat: add OAuth support for Claude Code tokens

Add support for Claude Code OAuth tokens (sk-ant-oat01-*) in the Anthropic provider.

Claude Code generates OAuth tokens that require different authentication headers than standard Anthropic API keys:
- Standard API keys use: x-api-key header
- OAuth tokens require: Authorization: Bearer header + anthropic-beta: oauth-2025-04-20 header

Changes:
- Add isOAuthToken() to detect Claude Code OAuth tokens by sk-ant-oat01- prefix
- Update NewProviderWithBaseURL() to use proper headers for OAuth tokens
- Update Chat() method to handle OAuth tokens when using tokenSource

This implementation matches the approach used in zeroclaw (Rust implementation).

Fixes authentication errors when using Claude Code OAuth tokens:
- Before: "OAuth authentication is currently not supported"
- After: OAuth tokens work correctly with proper headers

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

pkg/providers/anthropic/provider.go

@@ -25,6 +25,11 @@ type (
 
 const defaultBaseURL = "https://api.anthropic.com"
 
+// isOAuthToken detects Claude Code OAuth tokens by their prefix
+func isOAuthToken(token string) bool {
+	return strings.HasPrefix(strings.TrimSpace(token), "sk-ant-oat01-")
+}
+
 type Provider struct {
 	client      *anthropic.Client
 	tokenSource func() (string, error)
@@ -37,10 +42,22 @@ func NewProvider(token string) *Provider {
 
 func NewProviderWithBaseURL(token, apiBase string) *Provider {
 	baseURL := normalizeBaseURL(apiBase)
-	client := anthropic.NewClient(
-		option.WithAuthToken(token),
-		option.WithBaseURL(baseURL),
+
+	var opts []option.RequestOption
+
+	// OAuth tokens (Claude Code) require special handling
+	if isOAuthToken(token) {
+		opts = append(opts,
+			option.WithHeader("Authorization", "Bearer "+token),
+			option.WithHeader("anthropic-beta", "oauth-2025-04-20"),
 		)
+	} else {
+		opts = append(opts, option.WithAuthToken(token))
+	}
+
+	opts = append(opts, option.WithBaseURL(baseURL))
+
+	client := anthropic.NewClient(opts...)
 	return &Provider{
 		client:  &client,
 		baseURL: baseURL,
@@ -77,8 +94,17 @@ func (p *Provider) Chat(
 		if err != nil {
 			return nil, fmt.Errorf("refreshing token: %w", err)
 		}
+
+		// OAuth tokens (Claude Code) require special handling
+		if isOAuthToken(tok) {
+			opts = append(opts,
+				option.WithHeader("Authorization", "Bearer "+tok),
+				option.WithHeader("anthropic-beta", "oauth-2025-04-20"),
+			)
+		} else {
 			opts = append(opts, option.WithAuthToken(tok))
 		}
+	}
 
 	params, err := buildParams(messages, tools, model, options)
 	if err != nil {