fix(claude-code): add oauthToken config for subscription SDK auth

`claude login` alone does not authorize SDK / non-interactive
(`claude -p`) usage. Anthropic gates that behind a separate long-lived
token minted by `claude setup-token`. Without it, subscription users
hit `billing_error` from the SDK even though interactive Claude Code
works fine.

Changes:

- ClaudeCodeConfig.oauthToken: new optional field. When set, the
  provider exports it as CLAUDE_CODE_OAUTH_TOKEN before invoking the
  SDK, which makes the SDK bill against the user's Pro/Max
  subscription instead of API credits.
- apiKey continues to work for users with a console API key. If both
  are present, oauthToken takes precedence.
- billing_error / authentication_failed messages now point users at
  `claude setup-token` so the fix is obvious from the error alone.
- README: rewrite the Claude Code section to document both modes and
  the `claude setup-token` step explicitly.
- examples/claude-code.ts: read CLAUDE_CODE_OAUTH_TOKEN from env so
  the smoke test actually works for subscribers.

README.md

@@ -52,27 +52,46 @@ const claude = new ClaudeProvider({
 });
 ```
 
-### Claude Code (subscription via local CLI)
+### Claude Code (subscription or API key via local CLI)
 
-`ClaudeCodeProvider` wraps `@anthropic-ai/claude-agent-sdk`, which authenticates through the local `claude` CLI. This is the supported path for **Claude Pro / Max subscribers** who don't have a console API key.
+`ClaudeCodeProvider` wraps `@anthropic-ai/claude-agent-sdk`, which spawns the local `claude` CLI under the hood. It supports two billing modes:
 
-**Setup:** install the CLI and run `claude login` once. No API key required.
+**Mode A — Claude Pro/Max subscription** (billed against subscription credits):
+
+```bash
+# One-time: install the CLI, then mint a long-lived OAuth token.
+# `claude login` alone is NOT enough — Anthropic gates SDK use behind setup-token.
+claude setup-token
+```
 
 ```typescript
 import { ClaudeCodeProvider } from 'simple-ai-provider';
 
-const claude = new ClaudeCodeProvider({});           // uses local credentials
-await claude.initialize();
+const claude = new ClaudeCodeProvider({
+  oauthToken: process.env.CLAUDE_CODE_OAUTH_TOKEN  // from `claude setup-token`
+});
 
+await claude.initialize();
 const response = await claude.complete({
   messages: [{ role: 'user', content: 'Hello!' }]
 });
 ```
 
-You can still pass `apiKey` to override (it's set as `ANTHROPIC_API_KEY` for the SDK). Optional config:
+**Mode B — Console API key** (billed per-token):
+
+```typescript
+const claude = new ClaudeCodeProvider({
+  apiKey: process.env.ANTHROPIC_API_KEY
+});
+```
+
+If both are present, `oauthToken` wins. Either field can also be picked up from the environment (`CLAUDE_CODE_OAUTH_TOKEN` / `ANTHROPIC_API_KEY`) without being passed explicitly.
+
+Optional config:
 
 ```typescript
 new ClaudeCodeProvider({
+  oauthToken: '...',
   defaultModel: 'sonnet',  // 'sonnet' | 'opus' | 'haiku' | 'inherit' | full model ID
   maxTurns: 1,             // 1 for plain completion; raise for agent/tool loops
   allowedTools: [],        // tool names to enable (default: none)