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.

docs/extension-sdk.md

@@ -35,6 +35,8 @@ Reference implementations:
 | `ext.compose-ssh` | `compose_ssh.py` | Remote compose stack + tar snapshots |
 | `ext.vm-packer` | `vm_packer.py` | Attach workspace on pre-built VM |
 | `ext.saas-stub` | `saas_stub.py` | Metered stub + metadata snapshots |
+| `ext.e2b` | `e2b.py` | E2B cloud adapter |
+| `ext.modal` | `modal.py` | Modal cloud adapter |
 
 ## Registration
 
@@ -106,6 +108,6 @@ Implement `estimate_cost` and `meter_actual` on `SandboxExtension`. Register wit
 | Feature | Workplan |
 |---------|----------|
 | Packer build orchestration from `create` | Future WP |
-| E2B / Modal / Daytona cloud adapters | Post SAND-WP-0006 |
+| Daytona OSS cloud adapter | Future WP |
 | fin-hub billing export | Future |
 | Cross-host snapshot transfer | Future |