Add --ignore-errors option to spoom srb tc

[Morriar]

Summary

• Add --ignore-errors option to spoom srb tc to ignore known errors by matching error_code:file:line from an ignore file
• Default ignore file location: sorbet/ignored_errors.cfg (auto-detected when present)
• Ignored errors are hidden from output, shown as a count in the summary (e.g. Errors: 2 ignored, 7 total), and don't cause a non-zero exit code
• Supports # comments and blank lines in the ignore file

Ignore file format

# Comments supported
5002:errors/errors.rb:5
7003:errors/errors.rb:11

Test plan

• All errors ignored → exit 0, summary shows X ignored, Y total
• Partial match → exit 1, only non-ignored errors displayed
• Custom path via --ignore-errors=path
• Comments and blank lines in ignore file
• No ignore file → existing behavior unchanged (29 tests pass)