chore: add Copilot review instruction for extension tenant usage

Adds a path-scoped Copilot instruction for cli/azd/extensions/** that
flags use of Subscription.TenantId (resource tenant) instead of
Subscription.UserTenantId (user access tenant) for credential creation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: vhvb1989

.github/instructions/extensions.instructions.md

@@ -0,0 +1,8 @@
+applyTo:
+  - cli/azd/extensions/**
+---
+- When accessing a `Subscription` from `PromptSubscription()`, always use
+  `Subscription.UserTenantId` (user access tenant) for credential creation,
+  NOT `Subscription.TenantId` (resource tenant). For multi-tenant/guest users
+  these differ, and using `TenantId` causes authentication failures.
+  The `LookupTenant()` API already returns the correct user access tenant.