Multi-device simultaneous sessions broken since v1.0.15/1.0.16 — logging in on Device B automatically logs out Device A

### Describe the bug

## Bug Report

### Description
Since approximately v1.0.15–1.0.16, logging into Copilot CLI on a second device
automatically invalidates/logs out the session on the first device.

This worked correctly for months before this regression appeared.

### Steps to Reproduce
1. Log in on Device A → session works fine
2. Log in on Device B (same GitHub account)
3. Device A is immediately logged out / session invalidated

### Expected Behavior
Both devices should maintain independent, simultaneous active sessions
(as was the case before v1.0.15).

### Actual Behavior
New login on any device invalidates all other active sessions for the same account.

### Environment
- Copilot CLI version: 1.0.18
- OS: Windows
- Plan: Copilot Pro
- Auth method: OAuth device flow

### Additional Context
This is a regression. Multi-device usage worked fine for months prior to
the auth/user-switcher changes introduced in v1.0.15–1.0.16:
- 1.0.16: "MCP servers load correctly after login, user switch, and /mcp reload"
- 1.0.15: "User switcher and /user list display accounts in alphabetical order"

These changes appear to have introduced a side effect where sessions are
now treated as single-instance per account rather than per-device.

### Affected version

_No response_

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_