fix: Pin CI QA toolchain with mise

[lox]

Summary

• pin the QA toolchain in mise.toml and commit the generated mise.lock
• run Buildkite lint and test via mise#v1.1.1 so branch builds and main use the same pinned go and golangci-lint
• fix the current protocol/fetch test lint violations and update contributor docs to match the new toolchain flow

Why

The failing main build was a real golangci-lint failure, and the repo was not actually pinning that tool through mise.

On top of that, the QA group in .buildkite/pipeline.yml depended on base_image, which only exists on main. Branch builds used base_image_check, so QA was not wired consistently across branches.

This changes QA to install the pinned repo toolchain directly, which removes the branch/main split and makes the lint version explicit in-repo.

Impact

• contributors and CI use the same pinned go, golangci-lint, and go-cover-treemap
• branch builds now run the same lint and test setup as main
• the current main lint breakage is fixed in the same change

Validation

• mise run ci
• mise run build
• bk pipeline validate --file .buildkite/pipeline.yml