perf: improve file scanning and async input handling

The changes optimize file scanning by using a dedicated scan_dir utility with
gitignore support and improve async handling in context input processing:

- Replace vim.fn.glob with optimized scan_dir utility
- Add gitignore respect to file scanning
- Fix potential UI freezes by properly handling async context input
- Ensure scheduler is called before UI operations

Closes #690

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Author: deathbeam

lua/CopilotChat/config/contexts.lua

@@ -1,3 +1,4 @@
+local async = require('plenary.async')
 local context = require('CopilotChat.context')
 local utils = require('CopilotChat.utils')
 
@@ -54,10 +55,12 @@ return {
     description = 'Includes content of provided file in chat context. Supports input.',
     input = function(callback, source)
       local cwd = utils.win_cwd(source.winnr)
-      local files = vim.tbl_filter(function(file)
-        return vim.fn.isdirectory(file) == 0
-      end, vim.fn.glob(cwd .. '/**/*', false, true))
+      local files = utils.scan_dir(cwd, {
+        add_dirs = false,
+        respect_gitignore = true,
+      })
 
+      async.util.scheduler()
       vim.ui.select(files, {
         prompt = 'Select a file> ',
       }, callback)

lua/CopilotChat/init.lua

@@ -350,15 +350,17 @@ function M.trigger_complete(with_context)
   if with_context and vim.startswith(prefix, '#') and vim.endswith(prefix, ':') then
     local found_context = M.config.contexts[prefix:sub(2, -2)]
     if found_context and found_context.input then
-      found_context.input(function(value)
-        if not value then
-          return
-        end
+      async.run(function()
+        found_context.input(function(value)
+          if not value then
+            return
+          end
 
-        local value_str = tostring(value)
-        vim.api.nvim_buf_set_text(bufnr, row - 1, col, row - 1, col, { value_str })
-        vim.api.nvim_win_set_cursor(0, { row, col + #value_str })
-      end, state.source or {})
+          local value_str = tostring(value)
+          vim.api.nvim_buf_set_text(bufnr, row - 1, col, row - 1, col, { value_str })
+          vim.api.nvim_win_set_cursor(0, { row, col + #value_str })
+        end, state.source or {})
+      end)
     end
 
     return