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.

profiles/profile.compose-checkpoint.yaml

@@ -0,0 +1,31 @@
+id: profile.compose-checkpoint
+version: "1.0.0"
+extension: ext.compose-ssh
+isolation:
+  level: container
+network:
+  default: deny
+  egress: []
+workspace:
+  mode: remote-canonical
+  access: rw
+scope_default: session
+ttl:
+  default: 4h
+  max: 24h
+  idle_reap: null
+resources:
+  cpu: null
+  memory_mb: null
+setup:
+  instructions: "Use sandboxer snapshot/restore for workspace checkpoints."
+  secret_refs: []
+placement:
+  prefer: [sandboxer01]
+  fallback: [coulombcore]
+reachability:
+  tunnel: ops-bridge
+  identity: ops-warden
+metadata:
+  cost_class: self-hosted
+  latency_class: standard