Implement SAND-WP-0006: SaaS payments, routing, and ext.saas-stub

Add credits store, metering on create/destroy, extension routing resolver,
metered SaaS stub extension, burst/saas profiles, credits CLI, docs, and tests.

.claude/rules/stack-and-commands.md

@@ -39,6 +39,8 @@ Sandbox CLI (v0):
 sandboxer create                              # canary self-deploy (profile.sandbox-canary)
 sandboxer create --profile profile.compose-e2e --input repo=/path/to/repo
 sandboxer create --profile profile.vm-haskell-build --input vm=haskell-build --input repo=/path
+sandboxer create --profile profile.saas-stub    # metered SaaS stub
+sandboxer credits show / credits add 10.00
 sandboxer get <id>
 sandboxer list
 sandboxer destroy <id>