fix(antigravity-auth): address Oracle feedback - custom credentials, logging, docs

- Fix custom credentials to actually work in OAuth/refresh flows
  - oauth.ts: Add clientId/clientSecret parameters to buildAuthURL(), exchangeCode()
  - token.ts: Add clientId/clientSecret parameters to refreshAccessToken()
  - fetch.ts: Pass credentials to oauth/token functions
  - plugin.ts: Use closure cache for credentials, pass to all flows

- Unify console.* logging policy with ANTIGRAVITY_DEBUG guards
  - constants.ts: Document logging policy
  - tools.ts: Guard console.warn with ANTIGRAVITY_DEBUG
  - plugin.ts: Guard 4 console.error with ANTIGRAVITY_DEBUG

- Add explicit init.body type handling
  - fetch.ts: Check body type, pass-through non-string bodies
  - fetch.ts: Document body type assumption

- Document SSE buffering behavior
  - response.ts: Add warning that current implementation buffers
  - response.ts: Add TODO for future ReadableStream enhancement

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ai-todolist.md

@@ -8,7 +8,7 @@
 
 ## 현재 진행 중인 작업
 
-**모든 작업 완료** - ✅ Phase 1-4 완료 (14 tasks)
+**모든 작업 완료** - ✅ Phase 1-4 완료 (14 tasks) + Oracle 문서화 이슈 수정
 
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