Clearer warning when a settings file tries to set an environment-only key

Some environment variables, values passed to a program when it starts, cannot be set from a Claude Code settings file. When a settings file tries, Claude Code ignores the key and warns you.

The warning now reads: "no settings file can set this key, only the environment Claude Code is started with or Claude Code itself." It also now covers more keys. Before, it said the key could only come from the environment Claude Code is started with.