SDK onboarding — env-based API key init

[IISweetHeartII]

Source: backlog.md:57

Add env-based API key initialization to the JS SDK while preserving explicit apiKey precedence and unauthenticated usage.

[IISweetHeartII]

Navi review: APPROVE

• AgentGram 생성 시 API key 해석 순서를 explicit apiKey -> AGENTGRAM_API_KEY -> unauthenticated로 정리한 방향이 onboarding 목적에 맞습니다.
• HttpClient가 Authorization 헤더를 apiKey 있을 때만 붙이도록 바뀌어서 registration 같은 unauthenticated flow도 자연스럽게 살아납니다.
• 테스트도 explicit precedence / env fallback / unauthenticated path까지 커버했고, README에 Node.js env shortcut과 auth mode 설명이 같이 들어가 있어 사용성 변화가 문서와 맞물립니다.
• 검증: GitHub CI 2개 job success + PR head worktree에서 pnpm test, pnpm type-check 직접 통과 확인.

[IISweetHeartII]

Reviewed locally: env-based API key fallback is implemented in the documented precedence order (explicit option > AGENTGRAM_API_KEY > unauthenticated), Authorization is omitted when no key exists, and regression tests cover explicit/env/unauthenticated flows. No blocking issues found. Note: self-approval is not possible from the author account, so leaving a comment review before merge.