Release 0.2.8

Fixes #4: cargo install fails due to rsa/pkcs8 release-candidate churn.
Downgrades russh 0.57 → 0.55 (last stable rsa ^0.9 line).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Release 0.2.7

essh v0.2.6