feat(content): reparent disposition-ladder to mechanics.md; purge Duncarrow

Move rule.disposition-ladder into content/lore/mechanics.md and delete the
Duncarrow scaffolding (source + 23 generated JSON). Remove the redundant
test_duncarrow.py (routing/secrecy already covered by test_emit.py synthetic
fixtures); repoint a cosmetic slug-test id.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HuHRPE7VfppUJEaoGBEUqZ

content/world/npcs/harn-blackwood.json

@@ -1,18 +0,0 @@
-{
-  "id": "npc.harn-blackwood",
-  "knows": [
-    {
-      "fact_id": "rumor.travelers-go-missing",
-      "gate": "warm"
-    },
-    {
-      "fact_id": "fact.harn-daughter-left",
-      "gate": "warm"
-    }
-  ],
-  "related": [
-    "town.duncarrow"
-  ],
-  "start_disposition": "neutral",
-  "type": "person"
-}

content/world/npcs/mayor-oswin-crell.json

@@ -1,23 +0,0 @@
-{
-  "id": "npc.mayor-oswin-crell",
-  "knows": [
-    {
-      "fact_id": "rumor.elves-avoid-the-shrine",
-      "gate": "cold"
-    },
-    {
-      "fact_id": "secret.crell-runs-slave-trade",
-      "gate": "never"
-    },
-    {
-      "fact_id": "fact.crells-guard-acts-alone",
-      "gate": "never"
-    }
-  ],
-  "related": [
-    "person.mayor-oswin-crell",
-    "secret.crell-runs-slave-trade"
-  ],
-  "start_disposition": "neutral",
-  "type": "person"
-}

content/world/npcs/mera-fenn.json

@@ -1,27 +0,0 @@
-{
-  "id": "npc.mera-fenn",
-  "knows": [
-    {
-      "fact_id": "rumor.elves-avoid-the-shrine",
-      "gate": "neutral"
-    },
-    {
-      "fact_id": "rumor.travelers-go-missing",
-      "gate": "warm"
-    },
-    {
-      "fact_id": "fact.militia-never-investigates",
-      "gate": "warm"
-    },
-    {
-      "fact_id": "fact.crells-guard-acts-alone",
-      "gate": "trusted"
-    }
-  ],
-  "related": [
-    "town.duncarrow",
-    "fact.crells-guard-acts-alone"
-  ],
-  "start_disposition": "cold",
-  "type": "person"
-}