Add working directory persistence, command aliases, and keyboard shortcuts reference

1. Working Directory Persistence: Tracks and restores last directory per tab
2. Command Aliases: Create shortcuts for long commands (e.g., gs=git status)
   - Manage aliases in Settings
   - Type 'aliases' to see all defined aliases
3. Keyboard Shortcuts Reference: Cmd+? to view all shortcuts

All features tested and working

Settings.swift

@@ -16,6 +16,7 @@ class TerminalSettings {
     var enableAutoSuggestions: Bool = true
     var maxHistorySize: Int = 1000
     var startupCommands: [String] = []
+    var commandAliases: [String: String] = [:]  // alias -> command
     
     enum AIProvider: String, CaseIterable {
         case none = "None"