feat(justfile): add check-fmt and check recipes; pre-commit hook

Split concerns: fmt-check (check-fmt) from lint. check-fmt mirrors fmt
with each tool's --check/-d flag. check runs both as a single quality
gate. A new .githooks/pre-commit runs 'just check' on every commit;
bypass with git commit --no-verify.

Also drop just --fmt --check and prettier --check from the lint recipe
(they're format checks, belong in check-fmt).

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