Add comprehensive Subplan System specification defining module boundaries,
data models (Subplan, SubplanResult, SubplanTree), PostgreSQL schema with
indexes, the 8-step spawning algorithm, concurrency control via per-plan
semaphores, and error handling.
Implement invariant loading and enforcement in Strategize phase:
- Add InvariantViolationError exception class
- Add load_active_invariants() and check_invariants() methods to InvariantService
- Add _is_violation heuristic for action/invariant matching
- Add BDD tests with @load_invariants, @check_invariants, etc. tags
ISSUES CLOSED: #8725
HAL9000
bc6677b3cd