infra: add cargo-fmt-check pre-push prek hook (mirror CI)

The cargo-fmt failure on this PR slipped to CI because the pre-commit
prek hooks were never installed in the working clone. The existing
cargo-fmt hook reformats in place but only when it runs. Add a
pre-push cargo-fmt-check hook (`cargo fmt --all --check`) that mirrors
CI's Dagger `check` step exactly, so an unformatted commit is blocked
locally before it can reach the runner — even if the pre-commit hook was
skipped or not installed. Filtered to .rs pushes so Rust-free pushes pay
nothing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/changelog.d/prek-fmt-prepush.infra.md

@@ -0,0 +1 @@
+Added a `cargo-fmt-check` pre-push prek hook that runs `cargo fmt --all --check` (mirroring CI's Dagger `check` step) whenever a push touches a `.rs` file. The pre-commit `cargo-fmt` hook reformats in place, but only fires when installed and run; the pre-push check is a last-line guard so an unformatted commit can't reach the runner. Run `prek install --hook-type pre-push` to activate it.

prek.toml

@@ -102,6 +102,22 @@ language = "system"
 files = '\.rs$'
 pass_filenames = false
 
+# Pre-push safety net. The pre-commit cargo-fmt hook above reformats in place,
+# but only when it is installed and actually runs — a commit made before
+# `prek install`, or via a tool that skips hooks, can still carry unformatted
+# Rust to CI (where the Dagger `check` step runs `cargo fmt --all --check` and
+# fails). This hook re-checks at push time, mirroring CI byte-for-byte, so an
+# unformatted commit is blocked locally before it can reach the runner. It runs
+# only when the push range touches a .rs file, so Rust-free pushes pay nothing.
+[[repos.hooks]]
+id = "cargo-fmt-check"
+name = "cargo-fmt-check"
+entry = "cargo fmt --all --check"
+language = "system"
+files = '\.rs$'
+pass_filenames = false
+stages = ["pre-push"]
+
 # GitHub/Forgejo Actions workflow linting
 [[repos]]
 repo = "https://github.com/rhysd/actionlint"