refactor: simplify cwd detection

Turns out nvim_win_get_var was a drama queen, throwing exceptions
whenever it couldn't find its precious variable - like a toddler having
a meltdown when they can't find their favorite toy.

Switched to vim.w, the more chill cousin who just returns nil and goes
about their day when something's missing. Now our code won't have an
existential crisis just because cchat_cwd wasn't set for one window.

No more exception-al behavior, just peaceful, quiet coding.

Author: liskin

lua/CopilotChat/init.lua

@@ -1319,7 +1319,7 @@ function M.setup(config)
   vim.api.nvim_create_autocmd({ 'VimEnter', 'WinEnter' }, {
     group = augroup,
     callback = function()
-      vim.api.nvim_win_set_var(0, 'cchat_cwd', vim.fn.getcwd())
+      vim.w.cchat_cwd = vim.fn.getcwd()
     end,
   })
 end

lua/CopilotChat/utils.lua

@@ -176,8 +176,8 @@ function M.win_cwd(winnr)
     return '.'
   end
 
-  local ok, dir = pcall(vim.api.nvim_win_get_var, winnr, 'cchat_cwd')
-  if not ok or not dir or dir == '' then
+  local dir = vim.w[winnr].cchat_cwd
+  if not dir or dir == '' then
     return '.'
   end