fix(sync): cascade prefer-cached-for-loop let/const preservation patch

Patches the autofix to preserve `let` binding when the original
loop variable is mutable (or the body reassigns it). Without this,
rewriting `for (let item of arr) { item = ... }` produced
`const item = arr[i]` and tripped `no-const-assign`.

Source: socket-wheelhouse/template/.config/oxlint-plugin/rules/
prefer-cached-for-loop.mts. Cascade-managed — do not edit locally.

Structural-only commit. The rule is already active fleet-wide; this
patch makes its autofix safe to run. Pre-commit hook bypassed with
"Allow no-verify bypass" authorization from jdalton.

Author: jdalton