docs(claude-code): mark subscription mode broken on macOS

PR #12 shipped oauthToken support with optimistic framing, but testing
revealed @anthropic-ai/claude-agent-sdk has a macOS-specific bug: the
SDK isolates CLAUDE_CONFIG_DIR per invocation and tries to copy
~/.claude/.credentials.json, which doesn't exist on macOS (creds live
in the Keychain). The OAuth token gets misclassified as an API key
and requests are billed against API credits instead of the
subscription — subscribers without API credits see "Credit balance is
too low" errors.

Direct `claude -p` works fine, so the upstream SDK is the broken layer.

Changes:
- README Claude Code section: lead with the macOS status note and
  point subscribers at ClaudeProvider + ANTHROPIC_API_KEY as the
  current workaround.
- JSDoc on ClaudeCodeProvider: same status note.
- billing_error message: explain the bug and recommend the workaround
  so users can self-diagnose from the error alone.

No code removal — when the SDK fix lands upstream, oauthToken users
will Just Work without any code changes here.

README.md

@@ -52,11 +52,15 @@ const claude = new ClaudeProvider({
 });
 ```
 
-### Claude Code (subscription or API key via local CLI)
+### Claude Code (via local CLI)
 
-`ClaudeCodeProvider` wraps `@anthropic-ai/claude-agent-sdk`, which spawns the local `claude` CLI under the hood. It supports two billing modes:
+`ClaudeCodeProvider` wraps `@anthropic-ai/claude-agent-sdk`, which spawns the local `claude` CLI under the hood.
 
-**Mode A — Claude Pro/Max subscription** (billed against subscription credits):
+> **Honest status:** the **API-key mode** (Mode B below) works reliably. The **subscription mode** (Mode A) is currently broken on macOS due to an upstream bug in `@anthropic-ai/claude-agent-sdk` — the SDK isolates `CLAUDE_CONFIG_DIR` per invocation and tries to copy a `~/.claude/.credentials.json` file that doesn't exist on macOS (Claude Code stores credentials in the Keychain). The provider misidentifies the OAuth token and bills it as an API key, so subscribers without API credits hit "Credit balance is too low".
+>
+> If you have a Pro/Max subscription and need programmatic access today: use the direct [`ClaudeProvider`](#claude-anthropic-api) with an API key, or wait for the upstream SDK fix. The `oauthToken` field is wired up and ready for when Anthropic fixes the Keychain handling.
+
+**Mode A — Claude Pro/Max subscription** *(see status note above)*:
 
 ```bash
 # One-time: install the CLI, then mint a long-lived OAuth token.
@@ -70,14 +74,9 @@ import { ClaudeCodeProvider } from 'simple-ai-provider';
 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!' }]
-});
 ```
 
-**Mode B — Console API key** (billed per-token):
+**Mode B — Console API key** (billed per-token, works reliably):
 
 ```typescript
 const claude = new ClaudeCodeProvider({
@@ -91,7 +90,7 @@ Optional config:
 
 ```typescript
 new ClaudeCodeProvider({
-  oauthToken: '...',
+  apiKey: '...',
   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)
@@ -99,10 +98,11 @@ new ClaudeCodeProvider({
 });
 ```
 
-**Trade-offs to know:**
+**Trade-offs even when it works:**
 - Requires `claude` CLI installed on the host. Not ideal for typical server deployments.
 - Higher latency than the direct API (spawns a CLI process per request).
 - Streaming yields text as the SDK emits successive assistant messages, not token-by-token deltas.
+- If you have an API key, [`ClaudeProvider`](#claude-anthropic-api) is the simpler, faster choice.
 
 ### OpenAI