feat: snapshot/restore checkpoints (SAND-WP-0007)

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.

docs/snapshots.md

@@ -0,0 +1,47 @@
+# Workspace snapshots
+
+Point-in-time workspace checkpoints — SAND-WP-0007.
+
+## Overview
+
+Snapshots capture the remote workspace state of a **ready** sandbox without
+destroying it. Restore provisions a **new** sandbox from the checkpoint.
+
+| Operation | CLI | HTTP |
+|-----------|-----|------|
+| Create checkpoint | `sandboxer snapshot <sandbox_id>` | `POST /v1/sandboxes/{id}/snapshot` |
+| Restore | `sandboxer restore <snapshot_id>` | `POST /v1/snapshots/{id}/restore` |
+| List | `sandboxer snapshots list` | `GET /v1/snapshots` |
+| Get | `sandboxer snapshots get <id>` | `GET /v1/snapshots/{id}` |
+
+Snapshot metadata is stored at `~/.local/share/sandboxer/snapshots.json`.
+Extension artifacts (e.g. tarballs) live on the placement host.
+
+## Profile
+
+`profile.compose-checkpoint` binds `ext.compose-ssh` for checkpoint-enabled
+compose sandboxes. Use the same `inputs.repo` convention as `profile.compose-e2e`.
+
+## ext.compose-ssh behavior
+
+1. **Snapshot** — `tar czf` of `remote_dir` to `{base_dir}/snapshots/{id}.tar.gz`
+2. **Restore** — new `sandbox_id`, extract tarball, `compose up -d`
+
+Cross-host restore is not supported in v0 (artifact must be on the target host).
+
+## ext.saas-stub
+
+Metadata-only checkpoints for routing and payments tests. Restore reprovisions
+a fresh stub endpoint.
+
+## Extension contract
+
+Optional hooks on `SandboxExtension`:
+
+```python
+def supports_snapshots(self) -> bool: ...
+def snapshot(self, handle) -> dict[str, str]: ...
+def restore_from_snapshot(self, profile, snapshot_meta, inputs, host) -> dict[str, str]: ...
+```
+
+See `docs/extension-sdk.md`.