fix(tmux): stop layout override after spawn, use configured main pane size

Remove applyLayout(select-layout main-vertical) call after spawn which
was destroying grid arrangements by forcing vertical stacking. Now only
enforceMainPaneWidth is called, preserving the grid created by manual
split directions. Also fix enforceMainPaneWidth to use config's
main_pane_size percentage instead of hardcoded 50%.

src/features/tmux-subagent/action-executor-core.ts

@@ -72,7 +72,6 @@ export async function executeActionWithDeps(
 	)
 
 	if (result.success) {
-		await deps.applyLayout(ctx.config.layout, ctx.config.main_pane_size)
 		await enforceMainPane(ctx.windowState, ctx.config, deps)
 	}