feat: Sync auth between CLI and Coder Desktop

### Problem Description

Coder CLI and Coder Desktop (macOS/Windows) require separate sign-ins, leading to duplicate steps and inconsistent states. Desktop already handles token lifecycle in-app, but there’s no coordination with the CLI.

### Desired Solution

Enable a **unified sign-in experience** so logging in or out from either CLI or Desktop updates the other automatically, using OS-native secure storage and consistent state handling.

### Requirements

- **Single Sign-In Flow**
- Login via CLI → Desktop adopts session; login via Desktop → CLI adopts session.
- Keep existing CLI browser-based login (`/cli-auth`) unchanged.

- **Shared Secure Storage**
- **macOS:** System Keychain
- **Windows:** Windows Credential Manager
- One shared, canonical store per platform—avoid duplicates.

- **State Sync**
- Use a lightweight presence signal (e.g., file or metadata) to detect auth state changes at startup and periodically.
- Ensure both clients handle token refresh/expiry consistently.

- **Cross-Client Logout**
- Logout in one client clears session in the other within a short delay.

- **Multi-Deployment Support**
- Sync should be scoped per deployment/host to avoid cross-environment conflicts.

- **Migration**
- On first run, migrate existing credentials into the canonical store.

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- [ ] macOS
- [ ] Windows