Handle launch outside project directories

[Mirochill]

Fixes #375

Summary

• Check for project.yaml in the launch directory before running Docker preflight or creating a launch container.
• Print a direct error telling the user to run from an OpenSAFELY project directory or pass --directory.
• Update launch tests to use a temporary project directory and add coverage for the missing-project case.

Validation

• git diff --check
• git diff --check HEAD -- opensafely/launch.py tests/test_launch.py
• rg -n "is_project_directory|No project.yaml|OpenSAFELY project directory|project_dir|--directory" opensafely/launch.py tests/test_launch.py

Not run locally: pytest, test suite, build, lint, install, Docker, or any project command.