feat: add ClaudeCodeProvider for subscription users

Wraps @anthropic-ai/claude-agent-sdk so Claude Pro/Max subscribers (who
do not have a console API key) can use this library by authenticating
through the local `claude` CLI. Subscribers run `claude login` once and
the provider picks up the credentials automatically.

Provider behavior:
- query() is invoked in single-turn mode (maxTurns: 1, allowedTools: [])
  to behave as plain text completion by default. Both knobs are
  configurable for agentic use cases.
- Conversation history is flattened to a single prompt with role labels.
- doStream() emits text deltas as the SDK yields successive assistant
  messages. doComplete() accumulates and returns the final result.
- SDKAssistantMessageError codes (authentication_failed, billing_error,
  rate_limit, model_not_found, ...) map to AIErrorType variants.
- ClaudeCodeConfig.apiKey is optional — when omitted the SDK falls back
  to ANTHROPIC_API_KEY or local subscription credentials. The base
  validator is overridden to allow this.

Tradeoffs:
- Requires the `claude` CLI installed and logged in on the host. This
  is not suitable for typical server-side production deployments; it is
  the official path for subscription accounts.
- Higher latency (CLI process spawn) than the direct Anthropic API.
- The agent SDK is heavy. Bundle grows ~750KB; bun build now uses
  --target node so the SDK's Node built-in imports resolve correctly.

Wired into PROVIDER_REGISTRY ('claude-code'), createClaudeCodeProvider,
SUPPORTED_PROVIDERS, and re-exported from the package entry.

src/utils/factory.ts

@@ -6,13 +6,15 @@ import { ClaudeProvider, type ClaudeConfig } from '../providers/claude.js';
 import { OpenAIProvider, type OpenAIConfig } from '../providers/openai.js';
 import { GeminiProvider, type GeminiConfig } from '../providers/gemini.js';
 import { OpenWebUIProvider, type OpenWebUIConfig } from '../providers/openwebui.js';
+import { ClaudeCodeProvider, type ClaudeCodeConfig } from '../providers/claude-code.js';
 import { BaseAIProvider } from '../providers/base.js';
 
 export const PROVIDER_REGISTRY = {
   claude: ClaudeProvider,
   openai: OpenAIProvider,
   gemini: GeminiProvider,
-  openwebui: OpenWebUIProvider
+  openwebui: OpenWebUIProvider,
+  'claude-code': ClaudeCodeProvider
 } as const;
 
 export type ProviderType = keyof typeof PROVIDER_REGISTRY;
@@ -22,6 +24,7 @@ export interface ProviderConfigMap {
   openai: OpenAIConfig;
   gemini: GeminiConfig;
   openwebui: OpenWebUIConfig;
+  'claude-code': ClaudeCodeConfig;
 }
 
 export function createProvider<T extends ProviderType>(
@@ -59,3 +62,9 @@ export function createGeminiProvider(
 export function createOpenWebUIProvider(config: OpenWebUIConfig): OpenWebUIProvider {
   return new OpenWebUIProvider(config);
 }
+
+export function createClaudeCodeProvider(
+  options: Partial<ClaudeCodeConfig> = {}
+): ClaudeCodeProvider {
+  return new ClaudeCodeProvider(options);
+}