feat(worker): scaffold activities, workflows, worker entrypoint — T13

src/activity_core/activities.py:
  - load_activity_definition, resolve_context, log_run — @activity.defn
    stubs (raise NotImplementedError, bodies in T14–T17)

src/activity_core/workflows.py:
  - RunActivityWorkflow (orchestrator-tq) — @workflow.defn stub (T18)
  - TaskExecutorWorkflow (task-execution-tq) — @workflow.defn stub (T19)

src/activity_core/worker.py:
  - Connects to Temporal via TEMPORAL_HOST / TEMPORAL_NAMESPACE env vars
  - Spawns two Workers: orchestrator-tq and task-execution-tq
  - Runs until cancelled (python -m activity_core.worker)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/activity_core/activities.py

@@ -0,0 +1,41 @@
+"""Temporal activity definitions for activity-core.
+
+Activities run inside a Worker bound to 'orchestrator-tq'.
+Each function is decorated with @activity.defn and executed by
+RunActivityWorkflow via workflow.execute_activity().
+
+Implementations are added in T14–T17; stubs here let the worker
+register correctly before the bodies are written.
+"""
+
+from __future__ import annotations
+
+from temporalio import activity
+
+
+@activity.defn
+async def load_activity_definition(activity_id: str) -> dict:
+    """Load an ActivityDefinition row from the DB by ID.
+
+    Returns a JSON-serialisable dict (Pydantic .model_dump()).
+    Implemented in T14.
+    """
+    raise NotImplementedError("T14")
+
+
+@activity.defn
+async def resolve_context(context_sources: list[dict]) -> dict:
+    """Fetch and merge all context sources into a single snapshot dict.
+
+    Implemented in T15.
+    """
+    raise NotImplementedError("T15")
+
+
+@activity.defn
+async def log_run(run_payload: dict) -> str:
+    """Persist an ActivityRun record and return its run_id.
+
+    Implemented in T17.
+    """
+    raise NotImplementedError("T17")