refactor(schedule): add domain/ package files

Add the three files that complete stage 1 of the layered refactor:
- schedule/src/schedule/domain/__init__.py (empty package marker)
- schedule/src/schedule/domain/context.py
  (TERMINAL_NODE_STATES / FAILED_NODE_STATES / TERMINAL_RUN_STATES / naive_utc —
  pure types, no I/O)
- schedule/src/schedule/domain/execution.py
  (ExecutionResult dataclass, frozen=True)

The corresponding import-path rewrites in worker.py / orchestrator.py /
scheduler.py / execution.py were already landed in cdcfcb2 (the prior
commit on this branch). This commit only adds the missing domain/
package files those imports point at.

Validation:
- uv run --package schedule pytest schedule/tests -q: 18 passed
- uv run python -m compileall schedule/src: zero errors
- from schedule.domain.context / schedule.domain.execution importable
- main.py / pyproject.toml / tests/ unchanged

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

schedule/src/schedule/domain/context.py

@@ -0,0 +1,41 @@
+"""Shared constants and timezone helpers for the scheduler components.
+
+Designed to be import-side-effect-free: no logging, no I/O, no model imports.
+Used by ``scheduler``, ``orchestrator`` and ``worker`` modules.
+"""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime
+
+TERMINAL_NODE_STATES = frozenset({
+    "succeeded",
+    "failed",
+    "skipped",
+    "cancelled",
+    "timed_out",
+})
+FAILED_NODE_STATES = frozenset({"failed", "cancelled", "timed_out"})
+TERMINAL_RUN_STATES = frozenset({
+    "succeeded",
+    "failed",
+    "cancelled",
+    "timed_out",
+})
+
+
+def naive_utc(value: datetime | None) -> datetime | None:
+    """Normalize a datetime to naive UTC; pass through ``None``."""
+    if value is None:
+        return None
+    if value.tzinfo is None:
+        value = value.replace(tzinfo=UTC)
+    return value.astimezone(UTC).replace(tzinfo=None)
+
+
+__all__ = [
+    "FAILED_NODE_STATES",
+    "TERMINAL_NODE_STATES",
+    "TERMINAL_RUN_STATES",
+    "naive_utc",
+]

schedule/src/schedule/domain/execution.py

@@ -0,0 +1,21 @@
+"""Domain types for schedule node execution.
+
+Pure value objects — no I/O, no logging, no model imports. Safe to import
+from any layer.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class ExecutionResult:
+    status: str
+    exit_code: int | None
+    logs: bytes
+    result: bytes
+    result_file_name: str
+    result_content_type: str
+    error_code: str | None = None
+    error_message: str | None = None