Add --panel flag to new-split command (#10)

* Add --panel flag to new-split command

Allows splitting a specific panel without changing focus first.

Usage: cmuxterm new-split <direction> [--panel <id|index>]

Example: cmuxterm new-split down --panel 1

* Return new panel ID from new-split command

new-split now returns the UUID of the newly created panel, enabling
reliable chaining of split operations without index drift issues.

Before: OK
After:  OK F2675177-3838-49AF-A1A0-1744C0048E99

Example workflow to create left + 2x2 grid on right:
  RIGHT=$(cmuxterm new-split right | awk '{print $2}')
  BOTTOM=$(cmuxterm new-split down --panel $RIGHT | awk '{print $2}')
  cmuxterm new-split right --panel $RIGHT
  cmuxterm new-split right --panel $BOTTOM

CLI/cmuxterm.swift

@@ -192,10 +192,12 @@ struct CMUXCLI {
             print(response)
 
         case "new-split":
-            guard let direction = commandArgs.first else {
+            let (panelArg, remaining) = parseOption(commandArgs, name: "--panel")
+            guard let direction = remaining.first else {
                 throw CLIError(message: "new-split requires a direction")
             }
-            let response = try client.send(command: "new_split \(direction)")
+            let cmd = panelArg != nil ? "new_split \(direction) \(panelArg!)" : "new_split \(direction)"
+            let response = try client.send(command: cmd)
             print(response)
 
         case "list-panels":
@@ -509,7 +511,7 @@ struct CMUXCLI {
           ping
           list-tabs
           new-tab
-          new-split <left|right|up|down>
+          new-split <left|right|up|down> [--panel <id|index>]
           list-panels [--tab <id|index>]
           focus-panel --panel <id|index>
           close-tab --tab <id|index>

Sources/GhosttyTerminalView.swift

@@ -429,7 +429,7 @@ class GhosttyApp {
                 return false
             }
             return performOnMain {
-                tabManager.newSplit(tabId: tabId, surfaceId: surfaceId, direction: direction)
+                tabManager.newSplit(tabId: tabId, surfaceId: surfaceId, direction: direction) != nil
             }
         case GHOSTTY_ACTION_GOTO_SPLIT:
             guard let tabId = surfaceView.tabId,

Sources/TabManager.swift

@@ -764,9 +764,9 @@ class TabManager: ObservableObject {
         }
     }
 
-    func newSplit(tabId: UUID, surfaceId: UUID, direction: SplitTree<TerminalSurface>.NewDirection) -> Bool {
-        guard let tab = tabs.first(where: { $0.id == tabId }) else { return false }
-        return tab.newSplit(from: surfaceId, direction: direction) != nil
+    func newSplit(tabId: UUID, surfaceId: UUID, direction: SplitTree<TerminalSurface>.NewDirection) -> UUID? {
+        guard let tab = tabs.first(where: { $0.id == tabId }) else { return nil }
+        return tab.newSplit(from: surfaceId, direction: direction)?.id
     }
 
     func moveSplitFocus(tabId: UUID, surfaceId: UUID, direction: SplitTree<TerminalSurface>.FocusDirection) -> Bool {

Sources/TerminalController.swift

@@ -236,7 +236,7 @@ class TerminalController {
           ping                    - Check if server is running
           list_tabs               - List all tabs with IDs
           new_tab                 - Create a new tab
-          new_split <direction>   - Split focused surface (left/right/up/down)
+          new_split <direction> [panel] - Split surface (left/right/up/down), optionally specify panel
           list_surfaces [tab]     - List surfaces for tab (current tab if omitted)
           focus_surface <id|idx>  - Focus surface by ID or index (current tab)
           close_tab <id>          - Close tab by ID
@@ -311,24 +311,51 @@ class TerminalController {
         return "OK \(newTabId?.uuidString ?? "unknown")"
     }
 
-    private func newSplit(_ directionArg: String) -> String {
+    private func newSplit(_ args: String) -> String {
         guard let tabManager = tabManager else { return "ERROR: TabManager not available" }
 
-        let trimmed = directionArg.trimmingCharacters(in: .whitespacesAndNewlines)
-        guard let direction = parseSplitDirection(trimmed) else {
+        let trimmed = args.trimmingCharacters(in: .whitespacesAndNewlines)
+        let parts = trimmed.split(separator: " ", maxSplits: 1).map(String.init)
+        guard !parts.isEmpty else {
             return "ERROR: Invalid direction. Use left, right, up, or down."
         }
 
-        var success = false
+        let directionArg = parts[0]
+        let panelArg = parts.count > 1 ? parts[1] : ""
+
+        guard let direction = parseSplitDirection(directionArg) else {
+            return "ERROR: Invalid direction. Use left, right, up, or down."
+        }
+
+        var result = "ERROR: Failed to create split"
         DispatchQueue.main.sync {
             guard let tabId = tabManager.selectedTabId,
-                  let tab = tabManager.tabs.first(where: { $0.id == tabId }),
-                  let surfaceId = tab.focusedSurfaceId else {
+                  let tab = tabManager.tabs.first(where: { $0.id == tabId }) else {
                 return
             }
-            success = tabManager.newSplit(tabId: tabId, surfaceId: surfaceId, direction: direction)
+
+            // If panel arg provided, resolve it; otherwise use focused surface
+            let surfaceId: UUID?
+            if !panelArg.isEmpty {
+                surfaceId = resolveSurfaceId(from: panelArg, tab: tab)
+                if surfaceId == nil {
+                    result = "ERROR: Panel not found"
+                    return
+                }
+            } else {
+                surfaceId = tab.focusedSurfaceId
+            }
+
+            guard let targetSurface = surfaceId else {
+                result = "ERROR: No surface to split"
+                return
+            }
+
+            if let newPanelId = tabManager.newSplit(tabId: tabId, surfaceId: targetSurface, direction: direction) {
+                result = "OK \(newPanelId.uuidString)"
+            }
         }
-        return success ? "OK" : "ERROR: Failed to create split"
+        return result
     }
 
     private func listSurfaces(_ tabArg: String) -> String {