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/fixtures/policies/default/index.yaml

@@ -0,0 +1,8 @@
+name: default
+versions:
+  - id: v1
+    hash: testhashp1
+    author: Juan
+    message: fixture mínima
+    created_at: 2026-04-01T00:00:00Z
+active_version: v1

tests/fixtures/policies/default/versions/v1.yaml

@@ -0,0 +1,6 @@
+name: default
+version: v1
+description: Política mínima para tests del core (sin validadores)
+input_validators: []
+output_validators: []
+on_validator_error: fail_closed