Fix React Effect remount guidance

[lidorcg]

Problem

skills/.curated/react/references/conc-concurrent-safe.md recommends an empty-dependency Effect with a ref-guarded body for "once on mount" intent.

In development Strict Mode, React deliberately runs an extra setup → cleanup → setup cycle. A ref guard can suppress the second setup after cleanup, leaving the external synchronization torn down.

Proposed correction

Keep this change narrowly scoped to the canonical-resolution paragraph:

• synchronization with external systems uses an Effect with complete dependencies and cleanup that mirrors setup;
• app initialization runs at module scope or in a framework boot hook; and
• Effects remain correct across setup → cleanup → setup instead of suppressing remount setup with a ref guard.

Official references:

• https://react.dev/reference/react/useEffect
• https://react.dev/learn/synchronizing-with-effects
• https://react.dev/learn/you-might-not-need-an-effect