infra(prek): add cargo fmt as a pre-commit hook

Run `cargo fmt --all` in place over the workspace on any staged .rs change,
matching the repo's other in-place formatters (ruff-format, stylua, shfmt).
Unformatted Rust now fails the commit locally (it reformats + reports
"files were modified"), so the fmt-dirty commits that slipped through this
session can't recur. CI still enforces `cargo fmt --check` via Dagger as the
backstop. Verified: passes clean, catches + fixes a deviation.

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

prek.toml

@@ -83,6 +83,20 @@ repo = "https://github.com/JohnnyMorganz/StyLua"
 rev = "v2.4.1"
 hooks = [{ id = "stylua-system" }]
 
+# Rust formatting - cargo fmt over the whole workspace, in place (like the other
+# formatters above). Uses the system toolchain; CI also enforces it via
+# `dagger call check` (cargo fmt --check). Runs whenever a .rs file is staged.
+[[repos]]
+repo = "local"
+
+[[repos.hooks]]
+id = "cargo-fmt"
+name = "cargo-fmt"
+entry = "cargo fmt --all"
+language = "system"
+files = '\.rs$'
+pass_filenames = false
+
 # GitHub/Forgejo Actions workflow linting
 [[repos]]
 repo = "https://github.com/rhysd/actionlint"