fix(tmux): skip agent area width guard when 0 agent panes exist

When no agent panes exist, mainPane.width equals windowWidth, making
agentAreaWidth zero. The early return guard blocked initial pane creation
before the currentCount === 0 handler could execute.

Add currentCount > 0 condition so the guard only fires when agent panes
already exist, allowing the bootstrap handler to evaluate canSplitPane.

Closes #1939

src/features/tmux-subagent/spawn-action-decider.ts

@@ -32,7 +32,7 @@ export function decideSpawnActions(
 	)
 	const currentCount = state.agentPanes.length
 
-	if (agentAreaWidth < minPaneWidth) {
+	if (agentAreaWidth < minPaneWidth && currentCount > 0) {
 		return {
 			canSpawn: false,
 			actions: [],