Agent containers currently mount /tmp as a 3.2 GB tmpfs (RAM-backed).
Large scratch writes (nix-develop shells, cargo build dirs) eat container
RAM unnecessarily and can exhaust the tmpfs, losing in-flight work.
Set boot.tmp.useTmpfs = false so /tmp is disk-backed instead.
boot.tmp.cleanOnBoot (default true) preserves the ephemeral-per-boot
semantics — /tmp is cleared on container start, same as before.
Closes#2264. Supersedes PR #2299 (TMPDIR redirect + state/tmp).
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