Add explicit support for parallel execution of dependency-free subtasks:
- TeamPlan.get_independent_subtasks(): returns phases with depends_on==[]
(introspection entry — topological_sort already groups them in layer 0)
- TeamOrchestrator.MAX_INDEPENDENT_SUBTASKS=10: aligns with router.MAX_EXPERTS
- _rebalance_independent_subtasks(): when Lead over-decomposes (>10
independent subtasks), re-decompose once with merge hint. Fallbacks:
no gateway → keep original; LLM error → keep original; single-phase
fallback → keep original (don't collapse 11→1); still over → return
new (MAX_PHASES truncation handles it)
Tests: 15 new tests covering get_independent_subtasks, topological_sort
layer contract, SharedWorkspace path uniqueness, rebalance (5 paths),
and full execute() integration. Existing TeamOrchestrator tests pass.
Chiguyong