refactor: generic Manager<TBackend> in core

packages/core/src/index.ts

@@ -1,3 +1,5 @@
+export type { Backend, BackendTarget } from './backend.js'
+export { BackendDisposedError } from './backend.js'
 export type { ConnectionSource, ResolvedConnection, SshConfig } from './config/baseSchema.js'
 export { baseConnectionShape, sshConfigSchema } from './config/baseSchema.js'
 export type { NamedConfig, Registry, RegistryDeps } from './config/registry.js'
@@ -18,6 +20,8 @@ export {
     truncateJsonBudget,
     truncateRows
 } from './format.js'
+export type { ManagedConnection, Manager, ManagerDeps } from './manager.js'
+export { createManager } from './manager.js'
 export { errorMessage, fail, ok } from './mcp/respond.js'
 export type { Tunnel } from './net/tunnel.js'
 export { buildSshAuth, expandHome, openTunnel } from './net/tunnel.js'