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.

extensions/ext.e2b.yaml

@@ -0,0 +1,19 @@
+id: ext.e2b
+title: E2B cloud sandboxes
+description: >
+  Metered E2B Firecracker sandbox adapter. Requires E2B_API_KEY or OpenBao
+  secret_ref mapping at provision boundary.
+handler: sandboxer.extensions.e2b:E2BExtension
+capabilities:
+  isolation_levels: [microvm]
+  regions: [us, eu]
+  persistence: true
+  pricing_model: metered
+config:
+  provider: e2b
+  api_base: https://api.e2b.dev
+  api_key_env: E2B_API_KEY
+  secret_ref: e2b-api-key
+  template_id: base
+  rate_usd_per_hour: 0.15
+  session_fee_usd: 0.02