dsh-nushell-only

Nushell-only shell executor, dialect preflight, and Nushell syntax teaching for DeepSeek Harness (dsh 0.1.7-rc.1): every ctx.shell execution runs as nu --no-config-file -c, other shells are refused, bash/PowerShell habits are refused before they spawn (including the ones Nushell silently misreads, like 2>$null), a failed call carries an actionable Nushell hint verified against captured real Nushell stderr, and the model is taught Nushell.