Refactor MQTT course and bound backend message processing

Move lesson source into the teaching structure, align the course path with the learner's Go and DevOps experience, and add the interactive lesson guides. Add advanced DevOps lifecycle study as a separate section.

Bound MQTT message work and drain accepted jobs during shutdown; align the backend implementation with the revised course through Section 6.

docs/agents/domain.md

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+# Domain Docs
+
+This is a single-context repository. Engineering skills should use the root glossary and ADR directory when exploring domain concepts or decisions.
+
+## Before exploring
+
+- Read `GLOSSARY.md` at the repository root when it exists.
+- Read relevant decisions under `docs/adr/` when that directory exists.
+- If either is absent, proceed without flagging it or creating it just for setup. Domain-modeling work can create these documents when it resolves terminology or a decision.
+
+## Use the glossary's vocabulary
+
+When an output names a domain concept, use the term defined in `GLOSSARY.md`. If a needed concept is not defined, note the gap for domain-modeling work rather than inventing a competing term.
+
+## Flag ADR conflicts
+
+If a proposed change conflicts with an existing ADR, identify the ADR and explain the conflict instead of silently overriding it.