Make BottleBackend.prepare a template method that runs a cross-backend
_validate step (agent exists, named skills present on host, SSH
IdentityFiles resolve) and then delegates to a subclass-implemented
_resolve_plan for backend-specific resolution.
A future backend that overrides _resolve_plan can no longer forget to
validate skills or SSH keys; the validation runs unconditionally via
prepare. Backends with additional preconditions can override _validate
and chain via super().
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
didericis