Introduce an `azd update` command for manual upgrades

[kristenwomack]

Description

Add an azd update command that provides a clean, user‑friendly way to manually upgrade azd. The command should wrap existing install/upgrade mechanisms and improve the current copy‑paste script experience, while preserving safe fallback behavior if the upgrade fails.

In scope

• New azd update command
• Clear messaging when an update is available
• Execution of the same scripts users run manually today
• Failure messaging that preserves existing manual instructions

Out of scope

• Auto‑update
• Channel switching logic beyond messaging

Acceptance criteria

• Users can run azd update instead of copy/pasting scripts
• Failure does not leave azd in a broken state
• Clear fallback instructions are shown on failure
• No auto‑update behavior is introduced by this command