Improve error message when Docker is not running

[Nehal-cell]

Problem

When Docker is not running, CodeMorph crashes with a raw Docker error that is confusing for users.

Expected behaviour

CodeMorph should detect Docker is unavailable at startup and print a clear, actionable message:

❌ Docker is not running. CodeMorph requires Docker for test isolation.
   Start Docker Desktop and try again.

Files to change

• src/sandbox/docker_runner.py — wrap Docker connection check in try/except and raise a clean error