issue 800: keep notify's health as unit; the gate refuses a one-process module judged only by its own tool

ADR 0227 rule 8 / ADR 0240 rule 2 refuse a module with no health check of
another kind beside a tool check. mesh-mail-notifier has one process
(notify); wiring mail_notifier_health as its manifest health, as the review
note's wording suggested (mirroring mesh-mail's watch/mail_health), trips
that rule because mesh-mail also has keep (kind unit) beside watch.
mail_notifier_health stays built, tested and callable; notify's manifest
health stays kind unit until the lead decides how a one-process module
gets this automatically.

modules/mesh-mail-notifier/module.json

@@ -58,12 +58,7 @@
         "notify"
       ],
       "health": {
-        "kind": "tool",
-        "tool": "mail_notifier_health",
-        "interval": "60s",
-        "timeout": "10s",
-        "looks": 2,
-        "grace": "120s"
+        "kind": "unit"
       }
     }
   ],