build: stop swiftlint trailing-comma fixups conflicting with swift-format
The previous setup let `swiftlint --fix` enforce `trailing_comma:
mandatory_comma: true`, but swift-format 602 actively strips trailing
commas in multi-line collection literals whose last element is a
multi-line function call. The two tools end up oscillating, and any
`make check` would surface a flood of unrelated reformatting in
files that hadn't been touched.
- Disable swiftlint's `trailing_comma` rule and remove the
`mandatory_comma` block so swift-format is the sole authority on
trailing-comma placement.
- Drop `swiftlint --fix` from the `lint` Makefile target; lint
remains a pure check, formatting belongs to swift-format.