docs: add workaround for false-positive update banner on brew/winget

[SebTardif]

Summary

• adds a known-issue note for false-positive Update available banners on Homebrew and WinGet installs
• explains the version-channel mismatch (npm latest vs package manager registry lag)
• documents temporary workaround: DISABLE_AUTOUPDATER=1

Context

Fixes confusion reported in #18047 (and related duplicates) where users see an update prompt but brew upgrade or winget upgrade reports no newer package version.

Why docs

The public repository currently does not include the runtime updater implementation, so this change provides immediate user guidance while a runtime fix is developed in the appropriate codebase.

[SebTardif]

Corrected data point from my machine (Linux + Homebrew in WSL):

• At startup on Claude Code version 2.1.72, the app showed: Update available! Run: brew upgrade claude-code
• At that moment, this can still be a mismatch window where update guidance appears noisy
• Shortly after, Homebrew cask advanced and brew upgrade --cask claude-code upgraded 2.1.72 -> 2.1.73
• claude --version now reports 2.1.73 (Claude Code)

This supports the channel-lag hypothesis: npm/runtime may advertise newer availability before downstream package registries fully converge.

[Copilot]

Pull request overview

Adds user-facing documentation to reduce confusion when Claude Code shows an “Update available” prompt despite Homebrew/WinGet reporting no newer package version (issue #18047), by explaining the npm vs package-registry lag and providing a temporary opt-out.

Changes:

• Documented a known issue where update banners can be false-positive for Homebrew/WinGet installs.
• Explained the underlying version-channel mismatch (npm latest vs Homebrew/WinGet lag).
• Added a temporary workaround using DISABLE_AUTOUPDATER=1, with bash and PowerShell examples.

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[SebTardif]

Addressed the formatting review items in commit 3cf7b57:

• Updated heading style to match README conventions:
  • ## Known issue with false-positive update banner (Homebrew/WinGet)
• Expanded workaround safety guidance:
  • Added warning that DISABLE_AUTOUPDATER=1 also suppresses legitimate update notifications
  • Added explicit re-enable commands for both bash and PowerShell

Please re-review when convenient.

[SebTardif]

Scope cleanup: removed unrelated /commit-push-pr instruction/doc changes from this PR.

PR #33351 now contains only README.md updates related to issue #18047.