Fix arena gear lockout, extend DM window, scrub mining flavor

Arena helmet auto-equip now preserves the existing helmet to inventory
(or stashes the new drop if the existing piece is strictly higher tier),
and arena gear can be re-equipped via !adventure equip. Shop only blocks
swaps that aren't an upgrade over current arena tier. DM response window
bumped from 15m to 3h so NPC/treasure/shop prompts don't expire while
players are AFK. MiningSuccess flavor pools no longer name specific ores
in prose — uses {ore}/{location}/{tool} placeholders so the narration
matches the actual loot.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/adventure.go

@@ -46,7 +46,7 @@ func (p *AdventurePlugin) advUserLock(userID id.UserID) *sync.Mutex {
 	return val.(*sync.Mutex)
 }
 
-const advDMResponseWindow = 15 * time.Minute
+const advDMResponseWindow = 3 * time.Hour
 
 // advMarkMenuSent records that an actionable adventure menu was DM'd to the user.
 // Only bare-number DM replies within this window will be treated as adventure choices.
@@ -1108,7 +1108,7 @@ func (p *AdventurePlugin) checkTreasureDrop(userID id.UserID, char *AdventureCha
 			NewTreasure: drop.Def,
 			Existing:    existing,
 		},
-		ExpiresAt: time.Now().Add(5 * time.Minute),
+		ExpiresAt: time.Now().Add(advDMResponseWindow),
 	})
 
 	text := renderAdvTreasureDiscardPrompt(drop.Def, existing)