- retriever: add save_memory() writing to _box-drafts/ review queue, add
memory_dir param to retrieve() for isolated test routing
- handbook: new load_handbook_conventions() with safe no-op contract (returns
"" when dir absent/empty/unreadable, never raises)
- clarifier_llm: add context_provider=None seam to ClaudeClarifier and
build_claude_clarifier_callables(); failure in provider is silent
- planner: add context_provider=None seam to build_plan_prompt() and plan_node()
- coordinator: thread context_provider through default_plan_node_factory(),
only pass kwarg when non-None to preserve stub-monkeypatching in tests
- tests: 19 new WS5 tests covering all seams (1063 total, all passing)
