feat(mcp): cap read volume per caller identity

A scope authorises a read and says nothing about volume. An agent reads a
thousand records in seconds where a person clicks ten, and until now nothing
noticed: the token is valid, the tool is permitted, the ACL passes. That gap
stopped being theoretical the day an autonomous run started reading the whole
catalog on a schedule.

identifier: credential keys the counter on the sub claim — the end user,
whoever is asking on their behalf — so an agent looping under one person's
identity spends that person's budget and cannot spend everyone's. Verified on
catalog: pm-user and ro-user hold separate counters.

Per-route namespace, so a busy domain cannot exhaust another's.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kong/mcp/rate-limiting.yaml

@@ -18,7 +18,7 @@ plugin: rate-limiting-advanced
 config:
   identifier: credential
   # Per-route counters. Sharing a namespace across domains would let a busy
-  # catalog exhaust the budget for payments.
+  # domain exhaust the budget for another.
   namespace: ${NAME}-catalog-mcp
   strategy: local
   sync_rate: -1