add no-default-ignore for filesystem mode

[hyle]

Summary

Add --no-default-ignore to make filesystem ignore behavior explicit and user-controlled.

Changes

• Add --no-default-ignore to disable built-in default ignore patterns in filesystem mode
• Keep local .gitignore rules active when --no-default-ignore is used
• Apply the new behavior only in recursive/filesystem mode
• Update CLI help to include the new flag
• Update README option docs and ignore-behavior documentation
• Add CLI coverage proving:
  • built-in default ignores still apply by default
  • --no-default-ignore disables only the built-in defaults
  • local .gitignore rules still apply

Why

This makes the ignore model explicit in behavior, not just in documentation. Users can now choose whether filesystem mode should apply the built-in ignore defaults, without changing Git-backed or stdin-driven selection semantics.

Verification

• Ran make test successfully