refactor(backend): lift host-side validation onto the base class

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>

claude_bottle/backend/docker/provision/skills.py

@@ -1,10 +1,10 @@
 """Copy host-side skill directories into a running Docker bottle.
 
-Skills are validated on the host before launch by
-`DockerBottleBackend.validate_skills`; this module assumes that
-validation has already run. A skill disappearing between validation
-and copy still dies loudly rather than silently producing a partial
-container."""
+Skills are validated on the host before launch by the base class's
+`BottleBackend._validate_skills` (called from `prepare`); this module
+assumes that validation has already run. A skill disappearing between
+validation and copy still dies loudly rather than silently producing
+a partial container."""
 
 from __future__ import annotations