revert(nix): drop forced zsh in the devshell

[loss-and-quick]

Reverts #208.

Summary

#208 added an unconditional exec zsh to the devshell shellHook. The hook runs
on every entry into the devshell, not just an interactive nix develop, so
the exec also fires for nix develop --command …, direnv, and any scripted
use — it replaces the process before the command runs, so the command
silently never executes: zsh reads EOF and exits 0, a false green.

This broke nix develop --command:

• the rust CI job (clippy/test/codegen), silently a no-op since feat(nix): Force use zsh in devshell #208;
• the module (nightly + release) and desktop-linux jobs migrated to
  nix develop --command in ci: use Nix devshell for module builds and desktop-linux smoke #213 — they'd build nothing while reporting success.

Repro (on current main)

$ nix develop --command bash -c 'echo ran > /tmp/marker'
$ echo $?          # 0
$ cat /tmp/marker  # No such file or directory — the command never ran

Why revert rather than guard it

Forcing a shell switch in a shared devshell is opinionated and surprising even
when correctly gated: it drops developers into a shell they didn't ask for and
sources their ~/.zshrc on nix develop. Picking a shell is a per-developer
choice (direnv, nix develop -c zsh, a personal wrapper), not something the
shared flake — which CI and everyone else consume — should impose. Reverting
removes the whole class of problem instead of papering over it.

Verification

• nix develop --command … runs the command again (marker created, cargo from devshell)
• nix fmt nix/toolchain.nix — 0 changes
• nix build .#kasumi-desktop green (nix-build CI job)

Notes for reviewers

Recommend merging promptly — several CI jobs on main are currently silent no-ops.