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.

src/sandboxer/snapshots/__init__.py

@@ -0,0 +1,5 @@
+"""Snapshot checkpoint persistence."""
+
+from sandboxer.snapshots.store import SnapshotStore
+
+__all__ = ["SnapshotStore"]

src/sandboxer/snapshots/store.py

@@ -0,0 +1,47 @@
+"""Persistent snapshot index (JSON file)."""
+
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+
+from sandboxer.models import SnapshotRecord
+
+
+def _default_store_path() -> Path:
+    base = Path(os.environ.get("XDG_DATA_HOME", Path.home() / ".local" / "share"))
+    return base / "sandboxer" / "snapshots.json"
+
+
+class SnapshotStore:
+    def __init__(self, path: Path | None = None) -> None:
+        self.path = path or _default_store_path()
+        self.path.parent.mkdir(parents=True, exist_ok=True)
+
+    def _read(self) -> dict[str, dict]:
+        if not self.path.exists():
+            return {}
+        return json.loads(self.path.read_text())
+
+    def _write(self, data: dict[str, dict]) -> None:
+        self.path.write_text(json.dumps(data, indent=2, default=str))
+
+    def save(self, record: SnapshotRecord) -> None:
+        data = self._read()
+        data[record.snapshot_id] = record.model_dump(mode="json")
+        self._write(data)
+
+    def get(self, snapshot_id: str) -> SnapshotRecord | None:
+        raw = self._read().get(snapshot_id)
+        if not raw:
+            return None
+        return SnapshotRecord.model_validate(raw)
+
+    def list_all(self) -> list[SnapshotRecord]:
+        return [SnapshotRecord.model_validate(v) for v in self._read().values()]
+
+    def delete(self, snapshot_id: str) -> None:
+        data = self._read()
+        data.pop(snapshot_id, None)
+        self._write(data)