refactor(schedule): cleanup flat files + document layering (stage 6)

- Delete the six orphaned flat modules (context/executor/orchestrator/
  scheduler/storage_client/worker) — all import sites already point at the
  layered packages; keep notebook_runner.py as the compatibility shim
- Clear __pycache__; fix stale docstring module refs in surviving files
- Subpackage __init__.py files re-export public symbols per layer
  (CronScheduler / DispatchOrchestrator / SchedulerService / NodeExecutor /
  SchedulerStorageClient / ExecutionResult / TERMINAL_NODE_STATES ...)
- CLAUDE.md engineering notes: add "Schedule service layering" section
- Zero behavior change; schedule/pyproject.toml untouched

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -76,6 +76,15 @@ Hard-won lessons. Read the relevant bullet before touching the named area.
 - **Mock response ordering matters for re-reads.** `update_platform_employee` reads `current_role` before write then `response_role` after. A scalar mock returning a fixed value will return the pre-write role in the response — track call order or look up by `user.platform_role_id` post-write.
 - **Mock users must declare every attribute the handler writes.** `Users.deleted_at` is not in column defaults; `SimpleNamespace(user_id=...)` raises `AttributeError` on `target.deleted_at = now`. Set `user.deleted_at = None` explicitly.
 
+### Schedule service layering (domain / scheduling / application / execution / infrastructure)
+
+Lessons from the zero-behavior-change refactor that split the flat 11-file `schedule/src/schedule/` into five subpackages (commits 3118694 → c89132a). Public class names (`SchedulerService`, `CronScheduler`, `DispatchOrchestrator`, `NodeExecutor`, `SchedulerStorageClient`) and `schedule/pyproject.toml` are unchanged.
+
+- **`python -m schedule.notebook_runner` is a stable external contract.** The worker launches the notebook subprocess with `sys.executable, "-m", "schedule.notebook_runner"`. That `-m` string must never change — so `schedule/notebook_runner.py` survives as a 6-line shim re-exporting `main` from `schedule.execution.runners.notebook`. Don't "clean up" the shim.
+- **Mock `patch()` string targets and in-function lazy imports are invisible to import-line greps.** `test_janitor.py` patched `"schedule.orchestrator.session_scope"` and `test_worker.py` patched `"schedule.service.build_object_store"`; `worker.py` also had a `from schedule.service import ...` *inside a function body*. When files move, these silently no-op against the old path — and hard-crash once the old file is deleted. After any move, grep the whole tree for the old module path (including tests) and rewrite every hit, not just top-level imports.
+- **A new package dir shadows a same-named flat module.** Creating `schedule/execution/` makes the old `schedule/execution.py` silently dead code (the package wins import resolution), so move-then-delete, don't just copy. `git` usually detects these as renames, which keeps the diff reviewable.
+- **Docstring references survive file deletion.** After removing flat files, `:class:\`schedule.worker.NodeExecutor\``-style text can linger in docstrings and render as broken links. Grep for the old module name one more time at cleanup and rewrite comment-only refs too.
+
 ### Frontend state + routing (zustand + React Router v8)
 
 Lessons from splitting `frontend/app/features/platform/ModelPlatformApp.tsx` (1057 → 121 lines) into zustand stores + nested routes.