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/workflows.py

@@ -0,0 +1,57 @@
+"""Temporal workflow definitions for activity-core.
+
+Two workflows are registered here:
+  - RunActivityWorkflow  → orchestrator-tq
+  - TaskExecutorWorkflow → task-execution-tq
+
+Workflow IDs follow the conventions in docs/conventions.md:
+  RunActivityWorkflow:  activity-{activity_id}:{trigger_key}
+  TaskExecutorWorkflow: task-{run_id}:{task_type}:{index}
+
+Implementations are added in T18–T19; stubs here let the worker
+register and the type system resolve references in T14–T17.
+"""
+
+from __future__ import annotations
+
+from datetime import timedelta
+
+from temporalio import workflow
+
+with workflow.unsafe.imports_passed_through():
+    from activity_core.activities import (
+        load_activity_definition,
+        log_run,
+        resolve_context,
+    )
+
+_DEFAULT_TIMEOUT = timedelta(minutes=5)
+
+
+@workflow.defn
+class RunActivityWorkflow:
+    """Durable orchestration workflow.
+
+    Sequence (T18):
+      1. load_activity_definition(activity_id)
+      2. resolve_context(context_sources)
+      3. evaluate_templates(task_templates, context)  ← pure function, no activity
+      4. spawn TaskExecutorWorkflow child per template result
+      5. log_run(...)
+    """
+
+    @workflow.run
+    async def run(self, activity_id: str, trigger_key: str) -> dict:
+        raise NotImplementedError("T18")
+
+
+@workflow.defn
+class TaskExecutorWorkflow:
+    """Child workflow that executes one concrete task instance.
+
+    Stub implementation in T19.
+    """
+
+    @workflow.run
+    async def run(self, run_id: str, task_type: str, params: dict) -> dict:
+        raise NotImplementedError("T19")