Fix clearScreen/clearScrollback for split panes and update all help text

- Fixed Cmd+K crash by handling both tab and split pane modes
- Updated startup message to show Cmd+I for AI (not Cmd+K)
- clearScreen and clearScrollback now work with active pane
- All documentation now consistent with correct shortcuts

TerminalSession.swift

@@ -200,8 +200,9 @@ class TerminalSession: NSObject, NSTextViewDelegate, NSMenuDelegate {
             appendText("Features:\n")
             appendText("  • Cmd+T: New tab\n")
             appendText("  • Cmd+R: Search history (fuzzy search)\n")
-            appendText("  • Cmd+K: AI command generator\n")
+            appendText("  • Cmd+I: AI command generator\n")
             appendText("  • Cmd+P: Command snippets\n")
+            appendText("  • Cmd+K: Clear screen\n")
             appendText("  • Tab: Accept auto-suggestion\n")
             appendText("  • Type 'help' for more commands\n")
             appendText("━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━\n\n")