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

tools/content_build/tests/test_model.py

@@ -13,11 +13,11 @@ def _raw(**over) -> RawEntry:
 
 
 def test_namespace_and_slug():
-    e = entry_from_raw(_raw(id="npc.mera-fenn", type="person",
+    e = entry_from_raw(_raw(id="npc.specimen-teague", type="person",
                             start_disposition="cold",
                             knows=[{"fact": "rumor.x", "gate": "warm"}]))
     assert e.namespace == "npc"
-    assert e.slug == "mera-fenn"
+    assert e.slug == "specimen-teague"
 
 
 def test_category_predicates():