Add chat level perks: XP bonus, shop flavor, rare drop nudge, death pardon (Adventure 2.5 steps 3-6)

- XP bonus: +5% per 10 chat levels (cap +25% at 50+), applies to all
  adventure and arena XP
- Shop flavor: shopkeeper greeting changes at chat levels 10/30/50,
  replacing random flavor for recognized players
- Rare drop nudge: +0.5% per 10 chat levels (cap +2.5%) applied
  additively to treasure and masterwork drop rates
- Death pardon: chat level 20+ gets 33% chance to survive death once
  per 7 days (converts to bad-failure, quiet DM). Does not apply in
  arena. Fires before Sovereign Death's Reprieve check.
- New adventure_chat_perks.go with perk calculation helpers
- LastPardonUsed field + migration on adventure_characters

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/plugin/adventure_arena.go

@@ -361,12 +361,15 @@ func (p *AdventurePlugin) resolveArenaSurvival(ctx MessageContext, run *ArenaRun
 	run.RoundsSurvived++
 	run.LastMonster = monster.Name
 
-	// Award battle XP (Ironclad set: Battle-Hardened — +5% XP)
+	// Award battle XP (Ironclad set: Battle-Hardened — +5% XP, chat level bonus)
 	battleXP := tier.BattleXP
 	equip, _ := loadAdvEquipment(ctx.Sender)
 	if advEquippedArenaSets(equip)["ironclad"] {
 		battleXP = int(float64(battleXP) * 1.05)
 	}
+	if bonus := chatLevelXPBonus(p.chatLevel(ctx.Sender)); bonus > 0 {
+		battleXP = int(float64(battleXP) * (1.0 + bonus))
+	}
 	char.CombatXP += battleXP
 	leveled, newLevel := checkAdvLevelUp(char, "combat")
 	if leveled {