Add ClaudeProvider (anthropic-sdk-go) and CodexProvider (openai-go) that
use the correct subscription endpoints and API formats:
- CodexProvider: chatgpt.com/backend-api/codex/responses (Responses API)
with OAuth Bearer auth and Chatgpt-Account-Id header
- ClaudeProvider: api.anthropic.com/v1/messages (Messages API) with
Authorization: Bearer token auth
Update CreateProvider() routing to use new SDK-based providers when
auth_method is "oauth" or "token", removing the stopgap that sent
subscription tokens to pay-per-token endpoints.
Closes#18
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
