refactor(oauth): rename credentials/ to oauth/ for clarity

Separate OAuth credential reading (external CLI tools) from the main
CredentialManager (API Key configuration in credentials.json5).

- credentials.ts: API Key configuration management
- oauth/: External OAuth credential reading (Claude Code, Codex)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/cli/commands/chat.ts

@@ -17,7 +17,7 @@ import {
   getCurrentProvider,
   getLoginInstructions,
   type ProviderInfo,
-} from "../../credentials/providers.js";
+} from "../../oauth/providers.js";
 
 type ChatOptions = {
   profile?: string;