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_housing.go

@@ -217,7 +217,7 @@ func thomShopView(char *AdventureCharacter, balance float64) string {
 			sb.WriteString(fmt.Sprintf("   Closing costs (5%%): €%d\n", int(float64(def.BasePrice)*0.05)))
 			sb.WriteString(fmt.Sprintf("   Realtor fees (6%%): €%d\n", int(float64(def.BasePrice)*0.06)))
 			sb.WriteString(fmt.Sprintf("   **Total: €%d**\n\n", total))
-			sb.WriteString(fmt.Sprintf("Reply `buy` to purchase, or `buy <amount>` for a down payment.\n"))
+			sb.WriteString(fmt.Sprintf("`!thom buy` to purchase outright, or `!thom buy <amount>` for a down payment.\n"))
 		}
 	}