Add instance aware flow restriction to copilot-instructions.md (#8144)

Instance aware flow is a 1st party-only feature that should not be
suggested to 3rd party customers. This PR adds explicit guidance to
prevent Copilot from recommending it incorrectly.

## Changes

- **New section "Instance Aware Flow Restrictions"** in
`.github/copilot-instructions.md`:
- Marks `instanceAware` config and `instance_aware` query parameter as
1st party-only
  - Prohibits suggesting this feature for 3rd party applications
- Clarifies existing docs at `lib/msal-browser/docs/instance-aware.md`
are for internal use
- Provides alternative solutions for 3rd party multi-cloud scenarios
(standard flows, proper authority config, separate app registrations)

## Context

Instance aware flow enables dynamic cloud instance selection via STS
parameters in the `/authorize` response. While available in the public
API surface, it's designed exclusively for Microsoft internal
applications handling users across cloud boundaries.

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> Please create a PR for me that updates the "copilot-instructions.md"
file that states that the instance aware flow is only available for 1st
party customers, and not to suggest or use instance aware for 3rd party
customers. Please use the instructions located at
.github/copilot-instructions.md for the PR.


</details>



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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jo-arroyo <45890484+jo-arroyo@users.noreply.github.com>
Co-authored-by: Jo Arroyo <joarroyo@microsoft.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

.github/copilot-instructions.md

@@ -190,6 +190,19 @@ microsoft-authentication-library-for-js/
 - Write unit tests for new functionality and bug fixes
 - Document all public API changes
 
+### Instance Aware Flow Restrictions
+
+**IMPORTANT: The instance aware flow is not supported and should NEVER be suggested or used with this library**
+
+- **Do not suggest** the `instanceAware` configuration option or `instance_aware` query parameter
+- **Do not recommend** instance aware flow in code examples, documentation, or solutions
+- If asked about multi-cloud or cloud instance scenarios, provide alternative solutions that do not involve instance aware flow
+
+**What to Suggest Instead:**
+- Standard MSAL authentication flows without instance aware configuration
+- Proper authority configuration for the specific cloud environment the application targets
+- Separate application registrations for different cloud environments if needed
+
 ### Telemetry and Performance Monitoring
 
 **IMPORTANT: Add telemetry for any new operations or significant code paths in msal-browser and msal-common where observability would be useful.**