fix(config): propagate template through platform overrides

Problem: CpPlatformOverrides lacked a template field and merge_lang()
never copied ov.template into the effective language config, so
per-platform template overrides were silently dropped.

Solution: add template? to CpPlatformOverrides and forward it in
merge_lang(), matching how extension is handled.

lua/cp/config.lua

@@ -12,6 +12,7 @@
 ---@class CpPlatformOverrides
 ---@field extension? string
 ---@field commands? CpLangCommands
+---@field template? string
 
 ---@class CpPlatform
 ---@field enabled_languages string[]
@@ -259,6 +260,9 @@ local function merge_lang(base, ov)
   if ov.commands then
     out.commands = vim.tbl_deep_extend('force', out.commands or {}, ov.commands or {})
   end
+  if ov.template then
+    out.template = ov.template
+  end
   return out
 end