feat: cloud adapters E2B/Modal and billing export (SAND-WP-0010)

Add credentialed E2B and Modal extensions, burst routing fallback,
fin-hub meter export hook, BYOK docs, and 77 tests.

profiles/profile.modal-gpu.yaml

@@ -0,0 +1,32 @@
+id: profile.modal-gpu
+version: "1.0.0"
+extension: ext.modal
+isolation:
+  level: policy
+network:
+  default: deny
+  egress: []
+workspace:
+  mode: remote-canonical
+  access: rw
+scope_default: session
+ttl:
+  default: 2h
+  max: 8h
+  idle_reap: null
+resources:
+  cpu: null
+  memory_mb: null
+setup:
+  instructions: "Modal GPU/burst sandbox — requires MODAL_TOKEN_ID."
+  secret_refs: [modal-token-id]
+placement:
+  prefer: []
+  fallback: []
+reachability:
+  tunnel: ops-bridge
+  identity: ops-warden
+metadata:
+  cost_class: saas-metered
+  latency_class: low
+  observability: none