devcontainer-file is a read-only config file resource, not a running
container. Including it in CONTAINER_RESOURCE_TYPES causes
validate_container_available() to return True when only a config file
is linked, even though no actual container is available.
Closes#10598
Add ExecutionEnvironment enum (host/container) to domain models, implement
execution environment resolution with priority chain (tool > plan > project
> default), wire the tool runner to check execution_environment before
execution, and add CLI flags to plan use/execute and project context set.
When container is selected but no container resource is available, a clear
ContainerUnavailableError is raised with an actionable message.
Closes#512
HAL9000
freemo
