feat(providers): add SDK-based providers for subscription OAuth login

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>

go.mod

@@ -16,12 +16,15 @@ require (
 )
 
 require (
+	github.com/anthropics/anthropic-sdk-go v1.22.1 // indirect
 	github.com/go-resty/resty/v2 v2.17.1 // indirect
 	github.com/gogo/protobuf v1.3.2 // indirect
 	github.com/google/uuid v1.6.0 // indirect
+	github.com/openai/openai-go v1.12.0 // indirect
 	github.com/tidwall/gjson v1.18.0 // indirect
 	github.com/tidwall/match v1.2.0 // indirect
 	github.com/tidwall/pretty v1.2.1 // indirect
+	github.com/tidwall/sjson v1.2.5 // indirect
 	golang.org/x/crypto v0.48.0 // indirect
 	golang.org/x/net v0.50.0 // indirect
 	golang.org/x/sync v0.19.0 // indirect