Add --execution-env-priority flag to 'project context set' command,
enabling project-level execution environment priority per spec WF17.
- Add execution_env_priority field to ContextConfig domain model
- Validate flag value against ExecutionEnvPriority enum (fallback/override)
- Persist in context_policy_json, preserving existing execution_environment
- Display in 'project context show' Execution Environment section
- Merge with existing blob to avoid overwriting previously set fields
Tests: 9 Behave scenarios, 26 steps.
ISSUES CLOSED: #1079
