move response topic default off $SYS + reactive peer-mode design doc

[fabracht]

Summary

• Default responseTopicPrefix moved from $SYS/responses to $DB/clients. The MQTT 5 spec (§4.7.2) reserves $-prefixed topics for broker-internal use; production brokers (EMQX, HiveMQ, AWS IoT Core) reject client publishes under $SYS by default ACL. The field remains fully configurable — deployments depending on the old default can preserve behavior by setting responseTopicPrefix: '$SYS/responses' explicitly.
• Replaced the brittle response-topic regex parser in handleResponseMessage with a startsWith-based check. The old regex prepended \$ to the prefix, which only worked when the prefix started with $ — any non-$ override produced an invalid regex (\r was interpreted as carriage return).
• Added docs/design/reactive-peer-mode.md — design doc for an upcoming peer-coordinated sync mode that lets stitch run against a plain MQTT broker without MQDB. Not implemented; for review.

Breaking change

Default responseTopicPrefix changed. Documented in CHANGELOG.md under Unreleased. Deployments overriding the field explicitly are unaffected.

Test plan

• npm run check (typecheck + lint + format + 35 tests) passes
• node scripts/verify-changelog.mjs passes
• Verify against a frontend that currently overrides responseTopicPrefix explicitly — should be unaffected
• If a deployment relies on the old default, broker ACLs will need to accept $DB/clients/... (or set responseTopicPrefix: '$SYS/responses' to preserve)