docs(quota): edge + single-tenant local quota enforcement RFC (#171)

Brings the consolidated RFC onto the branch: it covers the two
non-management-plane deployments (edge/remote Milo and flat single-tenant local)
and corrects the assumption that quota enforcement implies mode==milo.

Also notes in the single-tenant sample config that Instance names must be unique
across cell namespaces, since flat ResourceClaims are keyed by instance-<name> in
one fixed project namespace (cross-namespace collision deferred, not fixed here).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: scotwells

config/samples/single-tenant-quota-config.yaml

@@ -34,6 +34,9 @@ data:
       # local quota is active and this is unset.
       localQuotaProjectID: my-project
       # Namespace where flat ResourceClaims are created. Defaults to "default".
+      # Because every claim lands here keyed by instance-<name>, Instance names
+      # must be unique across all cell namespaces: two same-named Instances in
+      # different namespaces would share one claim and under-count quota.
       localQuotaProjectNamespace: default
     featureFlags:
       # Leave off: local quota supersedes the edge forwarder (the direct watch