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/activities.py

@@ -15,6 +15,7 @@ import uuid
 from datetime import datetime, timezone
 
 from sqlalchemy import select
+from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 from temporalio import activity
 from temporalio.exceptions import ApplicationError
@@ -120,7 +121,11 @@ async def resolve_context(context_sources: list[dict]) -> dict:
 async def log_run(run_payload: dict) -> str:
     """Persist an ActivityRun record to Postgres and return its run_id.
 
+    Idempotent: uses INSERT … ON CONFLICT (run_id) DO NOTHING so Temporal
+    activity retries do not produce duplicate rows.
+
     Expected keys in run_payload:
+        run_id            (str UUID — computed deterministically in workflow)
         activity_id       (str UUID)
         scheduled_for     (ISO-8601 str or None)
         context_snapshot  (dict)
@@ -132,21 +137,28 @@ async def log_run(run_payload: dict) -> str:
     """
     Session = _get_session_factory()
 
+    run_id = uuid.UUID(run_payload["run_id"])
+
     scheduled_for: datetime | None = None
     if run_payload.get("scheduled_for"):
         scheduled_for = datetime.fromisoformat(run_payload["scheduled_for"])
 
-    row = ActivityRun(
-        activity_id=uuid.UUID(run_payload["activity_id"]),
-        scheduled_for=scheduled_for,
-        fired_at=datetime.now(tz=timezone.utc),
-        context_snapshot=run_payload["context_snapshot"],
-        tasks_spawned=run_payload["tasks_spawned"],
-        version_used=run_payload["version_used"],
+    stmt = (
+        pg_insert(ActivityRun)
+        .values(
+            run_id=run_id,
+            activity_id=uuid.UUID(run_payload["activity_id"]),
+            scheduled_for=scheduled_for,
+            fired_at=datetime.now(tz=timezone.utc),
+            context_snapshot=run_payload["context_snapshot"],
+            tasks_spawned=run_payload["tasks_spawned"],
+            version_used=run_payload["version_used"],
+        )
+        .on_conflict_do_nothing(index_elements=["run_id"])
     )
 
     async with Session() as session:
         async with session.begin():
-            session.add(row)
+            await session.execute(stmt)
 
-    return str(row.run_id)
+    return str(run_id)