feat(workers): runner loop + echo handler

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workers/tests/test_echo.py

@@ -0,0 +1,18 @@
+import json
+from void_workers.runner import Runner
+from void_workers.config import DATABASE_URL
+
+
+def test_echo_handler_runs(boss_ready):
+    boss_ready.execute(
+        "INSERT INTO pgboss.job(name, data) VALUES('echo', %s)",
+        (json.dumps({"ping": 42}),)
+    )
+    runner = Runner(dsn=DATABASE_URL, handlers={"echo": {"concurrency": 1}})
+    runner.run(once=True)
+    row = boss_ready.execute(
+        "SELECT state, output FROM pgboss.job WHERE name='echo' "
+        "ORDER BY created_on DESC LIMIT 1"
+    ).fetchone()
+    assert row[0] == "completed"
+    assert row[1] == {"pong": 42}