docs: add docs folder (architecture, developer, usage); update README; wire probe TTL cache + concurrency notes into docs

docs/usage/nats.md

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+# NATS Reference
+
+## Subjects
+- Aggregate events: tenant.<tenant>.aggregate.<type>.<id>
+- Workflow commands/events: shared helpers define exact formats
+
+## Headers (Producers)
+- tenant-id: required
+- Nats-Msg-Id: idempotency key (event_id, command_id, etc.)
+- x-correlation-id and correlation-id
+- traceparent and trace-id
+
+## Consumers
+- AckPolicy::Explicit
+- ack_wait: bounded timeout
+- max_deliver: bounded
+- max_ack_pending: aligned with concurrency