feat(api): implementa /executions con invoke + HITL approve/reject + persistencia

- POST /agents/{name}/invoke, GET /executions, GET /executions/{trace_id},
  POST /executions/{trace_id}/{approve,reject}.
- execution_index.json persiste trace_id → (agent_name, version) para reconstruir
  el contexto en approve/reject tras un reinicio.
- Refactor del orchestrator: split _snapshot → _build_execution + _snapshot, nuevo
  método público snapshot() (lee estado sin avanzar). Sustituye a los helpers
  _snapshot_execution/_build_graph_for_snapshot del plan, incompatibles con el
  checkpointer por-llamada.
- Aliases Annotated[...] de dependencias movidos a deps.py.
- Fixture mínima tests/fixtures/policies/default/.

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

tests/unit/test_runtime_orchestrator.py

@@ -2,6 +2,7 @@
 
 from datetime import UTC, datetime
 from pathlib import Path
+from uuid import uuid4
 
 import pytest
 
@@ -64,6 +65,23 @@ async def test_invoke_camino_feliz(orchestrator: AgentOrchestrator) -> None:
     ]
 
 
+async def test_snapshot_relee_estado_sin_avanzar(orchestrator: AgentOrchestrator) -> None:
+    agent = _agent()
+    paused = await orchestrator.invoke(
+        agent_def=agent, policy=_policy(), user_input="caída registros sip"
+    )
+    again = await orchestrator.snapshot(agent_def=agent, policy=_policy(), trace_id=paused.trace_id)
+    assert again is not None
+    assert again.status == "awaiting_approval"
+    assert again.trace_id == paused.trace_id
+    # Idempotente: leer no añade pasos al decision_path.
+    assert [s.step for s in again.decision_path] == [s.step for s in paused.decision_path]
+
+
+async def test_snapshot_trace_id_desconocido_devuelve_none(orchestrator: AgentOrchestrator) -> None:
+    assert await orchestrator.snapshot(agent_def=_agent(), policy=_policy(), trace_id=uuid4()) is None
+
+
 async def test_invoke_pausa_hitl_y_resume_aprueba(orchestrator: AgentOrchestrator) -> None:
     agent = _agent()
     paused = await orchestrator.invoke(