chore(repo): pin .go files to LF line endings via .gitattributes

[CalvinAllen]

Summary

Add a .gitattributes file pinning *.go files to LF line endings.

Why

On Windows with the default core.autocrlf=true, Git checks out .go files with CRLF, but gofmt always writes LF. Result: every gofmt run (including ./rnr check) leaves .go files appearing "modified" in git status. Staging them re-normalizes to the LF stored in HEAD, so the diff disappears, but the next gofmt brings the churn right back.

Pinning *.go to eol=lf tells Git to keep the working copy as LF on every platform, matching gofmt's output. The cycle stops.

Notes

• The Go files in this repo are already stored as LF in the object store (running git add --renormalize . was a no-op against .go), so this is purely a working-copy / checkout-time fix. No file content actually changes.
• Scope kept minimal: only .go files. Other file types can be added in follow-ups if they cause similar issues.

Test plan

• On a Windows checkout, run ./rnr check and confirm no .go files appear in git status afterwards.
• On macOS/Linux: confirm git status still clean after running ./rnr check.