feat(workflows): implement RunActivityWorkflow — T18

workflows.py — RunActivityWorkflow:
  1. load_activity_definition(activity_id)
  2. resolve_context(context_sources)
  3. evaluate_templates (pure, called in-workflow)
  4. log_run({run_id, ...}) — run_id = uuid5(NAMESPACE_URL, activity_id:trigger_key)
  5. start_child_workflow(TaskExecutorWorkflow, ...) per task spec
     ABANDON parent-close policy (fire-and-forget)
  Returns {"run_id": str, "tasks_spawned": int}

activities.py — log_run updated:
  - now accepts run_id in run_payload (deterministic, passed from workflow)
  - uses pg INSERT ... ON CONFLICT (run_id) DO NOTHING for idempotency

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

src/activity_core/workflows.py

@@ -7,16 +7,15 @@ Two workflows are registered here:
 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
 
+import uuid
 from datetime import timedelta
 
 from temporalio import workflow
+from temporalio.common import RetryPolicy
 
 with workflow.unsafe.imports_passed_through():
     from activity_core.activities import (
@@ -24,32 +23,107 @@ with workflow.unsafe.imports_passed_through():
         log_run,
         resolve_context,
     )
+    from activity_core.template_engine import evaluate_templates
 
-_DEFAULT_TIMEOUT = timedelta(minutes=5)
+_RETRY_POLICY = RetryPolicy(
+    initial_interval=timedelta(seconds=1),
+    backoff_coefficient=2.0,
+    maximum_interval=timedelta(minutes=5),
+    maximum_attempts=10,
+)
+
+_ACTIVITY_TIMEOUT = timedelta(minutes=5)
+_TASK_QUEUE = "task-execution-tq"
 
 
 @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(...)
+    Sequence:
+      1. load_activity_definition(activity_id)  → defn dict
+      2. resolve_context(defn.context_sources)  → context snapshot
+      3. evaluate_templates(templates, context) → task specs (pure, no activity)
+      4. log_run(...)                           → run_id
+      5. start_child_workflow per task spec     (fire-and-forget, detached)
     """
 
     @workflow.run
-    async def run(self, activity_id: str, trigger_key: str) -> dict:
-        raise NotImplementedError("T18")
+    async def run(
+        self,
+        activity_id: str,
+        trigger_key: str,
+        scheduled_for: str | None = None,
+    ) -> dict:
+        """
+        Args:
+            activity_id:    UUID of the ActivityDefinition row.
+            trigger_key:    ISO-8601 datetime (cron) or event_id (event trigger).
+                            Used as the idempotency key component.
+            scheduled_for:  ISO-8601 string of the nominal scheduled time (cron only).
+
+        Returns:
+            {"run_id": str, "tasks_spawned": int}
+        """
+        # ── 1. Load definition ────────────────────────────────────────────────
+        defn: dict = await workflow.execute_activity(
+            load_activity_definition,
+            activity_id,
+            start_to_close_timeout=_ACTIVITY_TIMEOUT,
+            retry_policy=_RETRY_POLICY,
+        )
+
+        # ── 2. Resolve context ────────────────────────────────────────────────
+        context_snapshot: dict = await workflow.execute_activity(
+            resolve_context,
+            defn["context_sources"],
+            start_to_close_timeout=_ACTIVITY_TIMEOUT,
+            retry_policy=_RETRY_POLICY,
+        )
+
+        # ── 3. Evaluate templates (pure — no activity) ────────────────────────
+        task_specs: list[dict] = evaluate_templates(
+            defn["task_templates"], context_snapshot
+        )
+
+        # ── 4. Log the run ────────────────────────────────────────────────────
+        # run_id is derived deterministically so log_run retries are idempotent.
+        run_id = str(
+            uuid.uuid5(uuid.NAMESPACE_URL, f"{activity_id}:{trigger_key}")
+        )
+        await workflow.execute_activity(
+            log_run,
+            {
+                "run_id": run_id,
+                "activity_id": activity_id,
+                "scheduled_for": scheduled_for,
+                "context_snapshot": context_snapshot,
+                "tasks_spawned": len(task_specs),
+                "version_used": defn["version"],
+            },
+            start_to_close_timeout=_ACTIVITY_TIMEOUT,
+            retry_policy=_RETRY_POLICY,
+        )
+
+        # ── 5. Spawn task executor children (fire-and-forget) ─────────────────
+        for index, spec in enumerate(task_specs):
+            child_id = f"task-{run_id}:{spec['task_type']}:{index}"
+            await workflow.start_child_workflow(
+                TaskExecutorWorkflow,
+                args=[run_id, spec["task_type"], spec["params"]],
+                id=child_id,
+                task_queue=_TASK_QUEUE,
+                parent_close_policy=workflow.ParentClosePolicy.ABANDON,
+            )
+
+        return {"run_id": run_id, "tasks_spawned": len(task_specs)}
 
 
 @workflow.defn
 class TaskExecutorWorkflow:
     """Child workflow that executes one concrete task instance.
 
-    Stub implementation in T19.
+    Stub implementation — T19.
     """
 
     @workflow.run