feat(copilot): load GitHub token from environment (#276)

* feat(copilot): load GitHub token from environment

The copilot module now first tries to load the GitHub token from the
environment variable 'GITHUB_TOKEN'. If the variable is not set, it
falls back to loading the token from the file.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* feat(copilot): Load GitHub token only in GitHub Codespaces

This commit modifies the get_cached_token function to load the GitHub
token from the environment only when running in GitHub Codespaces. This
change ensures that the token is not inadvertently loaded in other
environments.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: lingjie00

lua/CopilotChat/copilot.lua

@@ -91,6 +91,14 @@ local function find_config_path()
 end
 
 local function get_cached_token()
+  -- loading token from the environment only in GitHub Codespaces
+  local token = os.getenv('GITHUB_TOKEN')
+  local codespaces = os.getenv('CODESPACES')
+  if token and codespaces then
+    return token
+  end
+
+  -- loading token from the file
   local config_path = find_config_path()
   if not config_path then
     return nil