This change centralizes deployment state management and enables seamless
multi-deployment support. The extension now properly tracks per-deployment
credentials, syncs state across VS Code windows, and handles login/logout
flows in a unified way.

Key changes:

Architecture:
- Add DeploymentManager to centralize deployment state (url, label, token, user)
  and coordinate client updates, auth contexts, and workspace refreshes
- Add LoginCoordinator to handle login prompts with cross-window detection,
  preventing duplicate login dialogs when multiple windows need auth
- Move deployment types to src/deployment/ module with proper type guards

Storage & Auth:
- SecretsManager now stores per-deployment credentials using label-based keys
  (coder.session.<label>) instead of flat sessionToken storage
- Add LRU tracking for deployments with automatic cleanup of old credentials
- Add migration from legacy flat storage format
- Cross-window sync via secrets.onDidChange events
- Debug command (coder.debug.listDeployments) for inspecting stored state

Commands & Remote:
- Commands now use DeploymentManager instead of directly manipulating client
- Remote connection uses LoginCoordinator for auth prompts during workspace
  connections
- Remove forceLogout in favor of unified logout through DeploymentManager
- CliManager.configure() now called on every remote connection, with secrets
  storage as the source of truth for credentials

WebSocket improvements:
- CoderApi now implements Disposable to clean up WebSocket connections
- Add setCredentials() method to update host+token atomically, avoiding
  unnecessary reconnection cycles
- Add suspend() support to ReconnectingWebSocket for clean disconnects
- Simplify WebSocket fallback logic with cleaner SSE fallback handling

Tests:
- Update secretsManager tests for new per-deployment API
- Add comprehensive reconnectingWebSocket tests for suspend/reconnect
- Extend coderApi tests for credential handling