Add workspace checkpoint API with SnapshotStore, extension hooks on compose-ssh and saas-stub, manager orchestration, CLI/HTTP surface, profile.compose-checkpoint, and docs/tests.
- T10 smoke passed on CoulombCore (sand-boxer self-deploy, sandbox 4e542c51) - Add e2e smoke compose, scripts/smoke-compose-e2e.sh, make smoke-remote - Support SANDBOXER_COMPOSE_CMD for podman-compose hosts - FastAPI v0 stub at sandboxer.api.app; migration gaps doc - Mark workplan finished (all 10 tasks done)
