Update lua/opencode/ui/completion/subagents.lua

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: sudo-tee

lua/opencode/ui/completion/subagents.lua

@@ -10,7 +10,7 @@ local subagent_source = {
   complete = Promise.async(function(context)
     local subagents = require('opencode.config_file').get_subagents():await()
     local config = require('opencode.config')
-    local expected_trigger = '@'
+    local expected_trigger = config.get_key_for_function('input_window', 'mention') or '@'
     if context.trigger_char ~= expected_trigger then
       return {}
     end