Add domain models, application service, CLI integration, and full test
coverage for multi-project subplan orchestration.
New components:
- MultiProjectMetadata, ProjectScope, ProjectDependency domain models
- MultiProjectService for creating/managing multi-project plans
- CLI display of multi-project metadata in plan commands
- Behave BDD tests (20 scenarios), Robot Framework integration tests,
ASV benchmarks, and reference documentation
Also fixes pre-existing test flakiness in cli_core, core_cli_commands,
cli_plan_context_commands, and helper_server_stubs caused by environment
leakage and path-with-spaces issues.
ISSUES CLOSED: #199
