azd update: concurrent update locking

## Problem
No file locking on staging directory. Multiple concurrent `azd` instances could race — e.g. two terminals running `azd version` both trigger background staging, or a user runs `azd update` while auto-staging is in progress.

## Proposed
- Use file lock (e.g. `gofrs/flock`) on the staging directory during download and apply
- Skip staging if lock is already held (another instance is handling it)

## Context
Raised in design review #6910 and PR #6942 code review.

Parent epic: #6721