docs: add GitHub CI watcher usage guide to platform prompts

Both Claude and Codex platform prompts now include a short
section explaining when to use structured GitHub fields
(ci_provider, ci_repo, ci_run_id) for the host-driven fast
path instead of the generic LLM-per-tick watcher.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prompts/codex-platform.md

@@ -27,3 +27,12 @@ Your output is sent directly to the Discord group.
 - For CI/status/watch requests that require future follow-up, schedule `watch_ci`
 - Do not use generic recurring task registration from Codex
 - If the user wants a reminder or other non-CI recurring task, tell them to ask Claude/클코 to schedule it
+
+## CI 감시 (watch_ci)
+
+GitHub Actions run 감시는 structured 필드를 우선 사용:
+- ci_provider: "github", ci_repo: "owner/repo", ci_run_id: run ID
+- 이 조합 → host-driven fast path (LLM 토큰 소모 없음, 15초 polling)
+- structured 필드 없이 generic 등록 시 매 tick LLM 실행됨
+- ci_pr_number는 아직 미지원
+- GitHub 외 CI는 기존 generic 경로 사용