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.burst-sandbox.yaml

@@ -5,6 +5,8 @@ route:
   strategy: prefer-self-hosted
   extensions:
     - ext.compose-ssh
+    - ext.e2b
+    - ext.modal
     - ext.saas-stub
   max_cost_per_hour_usd: 1.0
 isolation:
@@ -25,8 +27,8 @@ resources:
   memory_mb: null
 setup:
   instructions: >
-    Prefer self-hosted compose on SANDBOXER_HOST; falls back to metered SaaS stub
-    when host is unavailable or SANDBOXER_FORCE_SAAS=1.
+    Prefer self-hosted compose on SANDBOXER_HOST; falls back to E2B/Modal when
+    credentials are configured, else metered saas-stub. Force SaaS: SANDBOXER_FORCE_SAAS=1.
   secret_refs: []
 placement:
   prefer: [sandboxer01]