feat(config): add templates.cursor_marker for post-template cursor placement

Problem: after apply_template writes a file's content to the buffer,
cursor positioning was left entirely to the user's setup_code hook,
forcing everyone to reimplement the same placeholder-stripping logic.

Solution: add an optional templates.cursor_marker config key. When set,
apply_template scans the written lines for the marker, strips it, and
positions the cursor there via bufwinid so it works in both the
provisional and existing-file paths.

lua/cp/config.lua

@@ -9,6 +9,9 @@
 ---@field commands CpLangCommands
 ---@field template? string
 
+---@class CpTemplatesConfig
+---@field cursor_marker? string
+
 ---@class CpPlatformOverrides
 ---@field extension? string
 ---@field commands? CpLangCommands
@@ -86,6 +89,7 @@
 ---@class cp.Config
 ---@field languages table<string, CpLanguage>
 ---@field platforms table<string, CpPlatform>
+---@field templates? CpTemplatesConfig
 ---@field hooks Hooks
 ---@field debug boolean
 ---@field open_url boolean
@@ -320,6 +324,13 @@ function M.setup(user_config)
     error('[cp.nvim] At least one platform must be configured')
   end
 
+  if cfg.templates ~= nil then
+    vim.validate({ templates = { cfg.templates, 'table' } })
+    if cfg.templates.cursor_marker ~= nil then
+      vim.validate({ cursor_marker = { cfg.templates.cursor_marker, 'string' } })
+    end
+  end
+
   vim.validate({
     hooks = { cfg.hooks, { 'table' } },
     ui = { cfg.ui, { 'table' } },