feat(world): annotate lamp/matches/letter for read/light/extinguish

Adds the new schema flags and per-state body sections so the dispatcher's
new verb handlers have content to narrate.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/world/items/letter.md

@@ -3,6 +3,10 @@ id: letter
 names: ["letter", "folded letter", "paper"]
 short: "a folded letter"
 takeable: true
+readable: true
 ---
 
+A folded letter. The wax seal has been broken once already.
+
+## read
 A folded letter on yellowed paper. The hand is unfamiliar. It reads: "Come at once. The thing in the cellar is waking."