Shared MCP Token Cache Across CLI Sessions

### Describe the feature or problem you'd like to solve

_No response_

### Proposed solution

# Feature Request: Shared MCP Token Cache Across CLI Sessions

## Problem

Each Copilot CLI terminal/session maintains its own independent MCP connections and token cache. When a token expires:

1. Running `/mcp reload` in **Terminal A** does NOT fix **Terminal B**
2. Each terminal must be individually refreshed
3. Users working with multiple CLI sessions (common for parallel workflows) must reload each one

This is inconsistent with how other developer tools work — `az login`, `gh auth login`, and `docker login` all share credentials across terminal sessions on the same machine.

## Proposed Solution

**Shared token cache**: MCP server OAuth tokens should be stored in a machine-local cache (e.g., OS keychain or a file-based token cache) that all CLI sessions on the same machine can read from.

### Implementation suggestions

- Store MCP OAuth tokens in a shared location:
- **Windows**: Windows Credential Manager or `~/.copilot/mcp-token-cache/`
- **macOS**: Keychain Access
- **Linux**: `libsecret` / `gnome-keyring` or file-based fallback
- When a CLI session needs an MCP token, check the shared cache first
- When any session refreshes a token (via `/mcp reload` or auto-refresh), update the shared cache
- Include a file lock or atomic write mechanism to prevent race conditions
- Optional: a `--no-shared-cache` flag for users who want isolated sessions

### Interaction with auto-refresh (Feature Request #1)

If auto-refresh is implemented, the shared cache becomes even more valuable:

- One session refreshes the token → all sessions benefit immediately
- No "thundering herd" of multiple sessions trying to refresh simultaneously

## Environment

- Copilot CLI on Windows (PowerShell)
- Multiple terminal sessions on the same machine
- MCP servers using Entra ID (Azure AD) OAuth

## Current Workaround

When tokens expire, users must run `/mcp reload` in **every open CLI session** individually. There is no way to refresh once and have all sessions pick up the new token.

### Example prompts or workflows

_No response_

### Additional context

_No response_

The issue names no files or tests. Start by tracing MCP token handling and the `/mcp reload` entry point; determine how sessions currently store and refresh tokens. Done means sessions share refreshed tokens safely across Windows, macOS, and Linux, with race prevention and the requested isolation option considered.

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