Add native binary fallbacks for Copilot CLI installation when npm is unavailable

[Copilot]

CLI shim scripts only used npm install to bootstrap Copilot CLI, which fails if npm isn't installed.

Windows (copilotCLIShim.ps1)

• Cascades: npm → winget (winget install GitHub.Copilot)
• Separate Install-CopilotCLI / Update-CopilotCLI functions check for npm availability before attempting, then fall back to winget

Linux/macOS (copilotCLIShim.ts)

• Cascades: npm → brew → curl → wget
• brew install copilot-cli, or the official install script via curl -fsSL https://gh.io/copilot-install | bash / wget -qO-
• Uses command -v (not which) for portable command detection
• Distinguishes npm update vs npm install for version upgrade vs fresh install paths

function installCopilotCLI(label: string, update = false): boolean {
    if (hasCommand('npm') && runNpm([update ? 'update' : 'install', '-g', PACKAGE_NAME], label)) { return true; }
    if (hasCommand('brew')) { if (runBrew(label)) { return true; } }
    if (hasCommand('curl')) { if (runCurl(label)) { return true; } }
    if (hasCommand('wget')) { if (runWget(label)) { return true; } }
    return false;
}

Error messages now link to the official docs page rather than showing npm-specific commands.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Downloading native binaries for Copilot CLI in terminal when copilot isn't installed</issue_title>
<issue_description>Today we install cli using npm install...
However this fails if user doesn't have npm,
We should use one of the native distributions for installing the cli binary, the installation instructions for various platforms can be found here https://github.com/github/copilot-cli?locale=en-US

We might want to update the code in src/extension/chatSessions/vscode-node/copilotCLIShim.ps1 and src/extension/chatSessions/vscode-node/copilotCLIShim.ts accordingly.

Edit shim scripts (the scripts that run in the termnial) that ensure Copilot CLI is installed before running:

• copilotCLIShim.ps1 (Windows): tries npm, then winget
• copilotCLIShim.ts (Linux/Mac): tries npm, brew, curl, then wget</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Downloading native binaries for Copilot CLI in terminal when copilot isn't installed vscode#294333

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[Copilot]

Pull request overview

Adds more resilient installation/update fallbacks for the Copilot CLI terminal shim scripts so bootstrap doesn’t depend solely on npm being present.

Changes:

• Linux/macOS shim: cascade installer attempts npm → brew → curl → wget, using command -v for portable command detection.
• Windows shim: splits install/update into dedicated functions and falls back from npm to winget.
• User-facing failure guidance now points to official Copilot CLI install docs instead of npm-specific commands.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
src/extension/chatSessions/vscode-node/copilotCLIShim.ts	Adds multi-strategy install/update logic for macOS/Linux (brew + official install script via curl/wget).
src/extension/chatSessions/vscode-node/copilotCLIShim.ps1	Adds winget fallback and separates install vs update behavior on Windows.