Format Go with gofmt, then golines -m 77 (closes #160)

REPO_POLICIES names gofmt then golines -m 77 as the Go formatter, says
what script/fmt, script/fmt-check and script/bootstrap do, and pins
golines v0.13.0 by commit hash. The Go style guide and both checklists
agree. lll stays at 88: golines leaves long strings and comments alone.

Model: opus-5-5

prompts/EXISTING_REPO_CHECKLIST.md

@@ -261,8 +261,8 @@ with your task.
 
 - [ ] Platform-standard formatter is configured (`black`, `prettier`, and for Go
       `gofmt` then `golines -m 77`)
-- [ ] Default formatter config, except four-space indents (not Go),
-      `proseWrap: always` for Markdown, and the Go 77-column limit
+- [ ] Default formatter config, except four-space indents (not Go) and the Go
+      77-column limit
 - [ ] Go: `script/bootstrap` installs `golines` with `go install` pinned to a
       commit hash; `script/fmt-check` fails when `gofmt` or `golines -m 77`
       would change a file