[Documentation] Please clarify that Claude Code's system prompt is not public

Unlike Claude.ai's system prompts (published at https://docs.anthropic.com/en/release-notes/system-prompts), Claude Code's system prompt is not publicly available anywhere in the documentation.

This causes confusion when users search for it or try to understand Claude Code's behavior (see #398, #2969).

Request: Could you add a note to the docs explicitly stating that Claude Code's system prompt is proprietary and not publicly available? For example:

> "Note: Claude Code's internal system prompt is not publicly available. Use CLAUDE.md files or --append-system-prompt to add custom instructions."

This would set clear expectations and reduce confusion.