reject no-default-ignore outside filesystem mode

[hyle]

Summary

Make --no-default-ignore behave honestly by rejecting it outside filesystem selection.

Changes

• Reject --no-default-ignore when used with:
  • --from-stdin
  • --staged
  • --unstaged
  • --diff
• Add a dedicated CLI error message explaining that the flag only applies to filesystem selection
• Update the README to state that constraint explicitly
• Add CLI coverage for both stdin-backed and Git-backed misuse cases

Why

--no-default-ignore only affects the recursive/filesystem path. Before this change, the CLI accepted it in explicit non-filesystem modes as a silent no-op, which made the contract less trustworthy. Rejecting it is more consistent with the rest of fuori’s flag validation style.

Verification

• Ran make test successfully