feat(cli): add third-party source note to update prompt

[liruifengv]

Related Issue

None — see the problem description below.

Problem

Homebrew is a third-party distribution channel that we do not manage, so new releases can arrive there with a delay. Users who installed via Homebrew see the update prompt and may run brew upgrade kimi-code before the formula catches up. The README also listed Homebrew as an install method, steering new users toward a channel we do not control.

What changed

• When the detected install source is Homebrew, the manual update message now adds a note that third-party sources may lag behind the latest release and points to the official installer (https://www.kimi.com/code). The same message is shared by the startup preflight and the kimi upgrade subcommand, so both surfaces pick it up.
• Removed the Homebrew install section from README.md and README.zh-CN.md, leaving the official install script as the only listed macOS/Linux method.
• Added test assertions for the new note on the Homebrew update-prompt path.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: fec7d8a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@fec7d8a

npx https://pkg.pr.new/@moonshot-ai/kimi-code@fec7d8a

commit: fec7d8a

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