Applies review finding #3. The trait wraps `std::fs::write`, which is
not atomic — `SIGINT` mid-write leaves a truncated file. Naming it
`FsWriteAtomic` promised a guarantee the implementation does not
deliver and would propagate to every downstream caller.

Drop the misleading suffix. The trait documents explicitly that it
inherits `std::fs::write`'s contract and nothing more. If a future
caller needs atomic semantics (write-tempfile-then-rename), it should
build that on top of `FsWrite` rather than hide the algorithm inside
the capability — keeping the trait minimal lets every callsite see
exactly what guarantees it inherits.

Mechanical rename across the cmd-shim crate and the package-manager
call sites; no behaviour change.