Add split panes feature and fix alias matching

- Split Panes: Cmd+D (vertical) and Cmd+Shift+D (horizontal)
- Close panes with Cmd+W when multiple panes exist
- Fixed alias matching to work with command arguments
- Aliases now match first word only (e.g., 'gs status' works with gs=git)

Split panes allow multiple terminal sessions side-by-side in the same tab

MainWindowController.swift

@@ -4,6 +4,9 @@ class MainWindowController: NSWindowController {
     var tabManager = TabManager()
     var tabBar: NSStackView!
     var contentView: NSView!
+    var splitView: NSSplitView?
+    var activePaneIndex: Int = 0
+    var panes: [TerminalSession] = []
     var historyPanel: HistorySearchPanel!
     var snippetPanel: SnippetPanel!
     var aiPanel: AIGeneratorPanel!
@@ -153,6 +156,10 @@ class MainWindowController: NSWindowController {
         fileMenu.addItem(NSMenuItem(title: "New Tab", action: #selector(addNewTab), keyEquivalent: "t"))
         fileMenu.addItem(NSMenuItem(title: "Close Tab", action: #selector(closeCurrentTab), keyEquivalent: "w"))
         fileMenu.addItem(NSMenuItem.separator())
+        fileMenu.addItem(NSMenuItem(title: "Split Pane Vertically", action: #selector(splitPaneVertically), keyEquivalent: "d"))
+        fileMenu.addItem(NSMenuItem(title: "Split Pane Horizontally", action: #selector(splitPaneHorizontally), keyEquivalent: "D"))
+        fileMenu.addItem(NSMenuItem(title: "Close Pane", action: #selector(closePane), keyEquivalent: "w"))
+        fileMenu.addItem(NSMenuItem.separator())
         fileMenu.addItem(NSMenuItem(title: "Clear Screen", action: #selector(clearScreen), keyEquivalent: "k"))
         fileMenu.addItem(NSMenuItem(title: "Clear Scrollback", action: #selector(clearScrollback), keyEquivalent: "K"))
         
@@ -500,6 +507,8 @@ class MainWindowController: NSWindowController {
         File:
           Cmd+T - New Tab
           Cmd+W - Close Tab
+          Cmd+D - Split Pane Vertically
+          Cmd+Shift+D - Split Pane Horizontally
           Cmd+K - Clear Screen
           Cmd+Shift+K - Clear Scrollback
         
@@ -531,6 +540,97 @@ class MainWindowController: NSWindowController {
         alert.runModal()
     }
     
+    @objc func splitPaneVertically() {
+        guard let currentSession = tabManager.getCurrentSession() else { return }
+        
+        // Create split view if it doesn't exist
+        if splitView == nil {
+            splitView = NSSplitView(frame: contentView.bounds)
+            splitView?.isVertical = true
+            splitView?.dividerStyle = .thin
+            splitView?.autoresizingMask = [.width, .height]
+            
+            // Move current session to split view
+            currentSession.scrollView.removeFromSuperview()
+            splitView?.addArrangedSubview(currentSession.scrollView)
+            panes = [currentSession]
+        }
+        
+        // Create new pane
+        let newSession = TerminalSession(title: "Pane \(panes.count + 1)")
+        newSession.setupTextView(frame: CGRect(x: 0, y: 0, width: contentView.bounds.width / 2, height: contentView.bounds.height))
+        newSession.startShell()
+        
+        splitView?.addArrangedSubview(newSession.scrollView)
+        panes.append(newSession)
+        activePaneIndex = panes.count - 1
+        
+        // Add split view to content if not already there
+        if splitView?.superview == nil {
+            contentView.subviews.forEach { $0.removeFromSuperview() }
+            contentView.addSubview(splitView!)
+        }
+        
+        newSession.textView.window?.makeFirstResponder(newSession.textView)
+    }
+    
+    @objc func splitPaneHorizontally() {
+        guard let currentSession = tabManager.getCurrentSession() else { return }
+        
+        // Create split view if it doesn't exist
+        if splitView == nil {
+            splitView = NSSplitView(frame: contentView.bounds)
+            splitView?.isVertical = false
+            splitView?.dividerStyle = .thin
+            splitView?.autoresizingMask = [.width, .height]
+            
+            // Move current session to split view
+            currentSession.scrollView.removeFromSuperview()
+            splitView?.addArrangedSubview(currentSession.scrollView)
+            panes = [currentSession]
+        }
+        
+        // Create new pane
+        let newSession = TerminalSession(title: "Pane \(panes.count + 1)")
+        newSession.setupTextView(frame: CGRect(x: 0, y: 0, width: contentView.bounds.width, height: contentView.bounds.height / 2))
+        newSession.startShell()
+        
+        splitView?.addArrangedSubview(newSession.scrollView)
+        panes.append(newSession)
+        activePaneIndex = panes.count - 1
+        
+        // Add split view to content if not already there
+        if splitView?.superview == nil {
+            contentView.subviews.forEach { $0.removeFromSuperview() }
+            contentView.addSubview(splitView!)
+        }
+        
+        newSession.textView.window?.makeFirstResponder(newSession.textView)
+    }
+    
+    @objc func closePane() {
+        guard panes.count > 1, activePaneIndex < panes.count else { return }
+        
+        let pane = panes[activePaneIndex]
+        pane.cleanup()
+        pane.scrollView.removeFromSuperview()
+        panes.remove(at: activePaneIndex)
+        
+        if panes.count == 1 {
+            // Only one pane left, remove split view
+            splitView?.removeFromSuperview()
+            splitView = nil
+            
+            let remainingSession = panes[0]
+            remainingSession.scrollView.removeFromSuperview()
+            remainingSession.scrollView.frame = contentView.bounds
+            contentView.addSubview(remainingSession.scrollView)
+            panes = []
+        }
+        
+        activePaneIndex = min(activePaneIndex, panes.count - 1)
+    }
+    
     @objc func clearHistory() {
         let alert = NSAlert()
         alert.messageText = "Clear Command History?"