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_npcs.go

@@ -158,7 +158,7 @@ func (p *AdventurePlugin) npcFireEncounter(userID id.UserID, npc string) {
 	p.pending.Store(string(userID), &advPendingInteraction{
 		Type:      "npc_encounter",
 		Data:      npc,
-		ExpiresAt: time.Now().Add(5 * time.Minute),
+		ExpiresAt: time.Now().Add(advDMResponseWindow),
 	})
 
 	if err := p.SendDM(userID, prompt); err != nil {