refactor: explicit initialization and dependency injection

- Replace init() with config.Load() function when loading env variables
- Inject portRegistry into session, server, and lifecycle structs
- Inject sessionRegistry directly into interaction and lifecycle
- Remove SetSessionRegistry function and global port variables
- Pass ssh.Conn directly to forwarder constructor instead of lifecycle interface
- Pass user and closeFunc callback to interaction constructor instead of lifecycle interface
- Eliminate circular dependencies between lifecycle, forwarder, and interaction
- Remove setter methods (SetLifecycle) from forwarder and interaction interfaces

session/lifecycle/lifecycle.go

@@ -51,7 +51,6 @@ func New(conn ssh.Conn, forwarder Forwarder, slugManager slug.Slug, port portUti
 
 type Lifecycle interface {
 	Connection() ssh.Conn
-	Channel() ssh.Channel
 	PortRegistry() portUtil.Registry
 	User() string
 	SetChannel(channel ssh.Channel)
@@ -69,10 +68,6 @@ func (l *lifecycle) User() string {
 	return l.user
 }
 
-func (l *lifecycle) Channel() ssh.Channel {
-	return l.channel
-}
-
 func (l *lifecycle) SetChannel(channel ssh.Channel) {
 	l.channel = channel
 }