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.

AGENTS.md

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+## Agent skills
+
+### Issue tracker
+
+Issues are tracked in `.scratch/`; Gitea Issues may also be used alongside it. See `docs/agents/issue-tracker.md`.
+
+### Triage labels
+
+Use the default labels: `needs-triage`, `needs-info`, `ready-for-agent`, `ready-for-human`, and `wontfix`. See `docs/agents/triage-labels.md`.
+
+### Domain docs
+
+Use the single-context layout. See `docs/agents/domain.md`.