ISSUE_ENGINEERING_LOOP_19: Loop canary: document notification smoke path (#21)

Co-authored-by: hyrule-engineering-loop[bot] <engineering-loop@as215932.net>

Author: hyrule-engineering-loop[bot]

docs/engineering-loop/notification-smoke-path.md

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+# Engineering Loop notification smoke path
+
+Each completed daemon cycle reports its outcome through two channels:
+
+1. **Discord webhook** — a short summary showing the run result
+   (success, stuck, or over-budget), the change class, the risk tier,
+   and iteration/wall-clock stats.
+2. **Icinga passive check** — the daemon submits `loop!engineering-loop`
+   with the corresponding state:
+   - `OK` when the cycle completed and produced a draft PR
+   - `WARNING` when the cycle is stuck or exhausted its retry budget
+   - `CRITICAL` when the cycle failed outright or breached a hard gate
+
+These are the Engineering Loop's own heartbeat/observability channels.
+They do not replace the per-repo CI gates or the authoritative role
+evaluators; they exist so the NOC can detect a silent or looping
+daemon without reading pull-request bodies.