fix: memory-lint agents were missing from the agents page

Both metas declared "strength": "medium", which is not an LlmStrength
(very_low | low | average | high | very_high). `discover()` warns and skips a
meta.json it cannot deserialize — deliberately, so one bad file does not blank
the whole roster — so the two agents never reached /api/agents and the page's
"system" section only ever showed event-triage.

The skip is right; its silence is not. `agents::tests::every_shipped_agent_meta_parses`
deserializes every shipped meta.json, so a typo'd field now fails the build
instead of quietly costing an agent its place in the UI.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

agents/memory-lint-private/meta.json

@@ -16,5 +16,5 @@
   "inject_skills": false,
   "inject_memory": ["user-memory/index.md"],
   "icon": "icon.png",
-  "strength": "medium"
+  "strength": "average"
 }

agents/memory-lint-shared/meta.json

@@ -16,5 +16,5 @@
   "inject_skills": false,
   "inject_memory": ["shared-memory/index.md"],
   "icon": "icon.png",
-  "strength": "medium"
+  "strength": "average"
 }