[DOCS] README does not clarify that source code is not included in this repository

## Problem

The README (and the repository in general) does not mention anywhere that this repository does not contain the source code of Claude Code.

A developer visiting https://github.com/anthropics/claude-code would reasonably expect to find the CLI source code here, but the repository only contains:

• Installation instructions
• Documentation links
• Plugins
• Bug reporting / privacy info

There is no note clarifying that Claude Code is closed source and that the CLI binary is distributed separately.

This has been raised before in #19073 (closed as "not planned") and is related to #22002. Neither addresses the simple documentation gap of clarifying this
directly in the README.

## Suggested fix

Add a note near the top of the README, for example:

> Note: This repository does not contain the source code of Claude Code. The CLI is proprietary and distributed as a pre-built binary. This repository
is used for issue tracking, changelogs, plugins, and documentation.

## Impact

This causes confusion for developers who want to:

• Contribute to the project
• Understand how the tool works internally
• Audit the code for security purposes

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Issue text drafted by Claude Code (claude-sonnet-4-6) based on an idea by the repository user.