azd update does not support daily builds for winget installations

## Problem

When `azd` is installed via `winget install Microsoft.Azd`, running `azd update` to switch to the daily channel is explicitly blocked. The code at `cmd/update.go:193-209` checks `IsPackageManagerInstall()` (winget/choco) and returns an error suggesting the user uninstall first.

Homebrew has full channel support — it can switch between `azd` and `azd@daily` casks seamlessly. Winget/Chocolatey lack this parity.

## Current workaround for winget daily update

The stable flow (`winget install Microsoft.Azd` → `azd update`) works because `azd update` downloads the MSI and installs it (via the PowerShell script). However, upgrading across channels with the MSI is unreliable and may fail unpredictably. The deterministic workaround is to uninstall first (`winget uninstall Microsoft.Azd`), then install daily via the PowerShell script.

Discussed offline with @danieljurek. We will follow up with winget daily build support later.