`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.
