Create default gooseignore file when missing

[mattjoyce]

Summary

• automatically create .gooseignore the first time we need the built-in safety patterns so the defaults are visible/
  editable instead of hidden in code
• share those defaults through a single constant and cover them with an explicit test

Testing

• cargo fmt
• cargo test -p goose-mcp test_default_patterns_when_no_ignore_files

[blackgirlbytes]

love that the default is there to keep average users from messing up their secrets files, but if theyre more advanced..they have the option to edit the ignore file.

[mattjoyce]

Thinking about it, we can help user more. I have added some guidance into the default .gooseignore, including some other examples of files that users might unintentionally expose.