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_treasure.go

@@ -241,11 +241,12 @@ var advAllTreasures = map[int][]AdvTreasureDef{
 
 // ── Treasure Drop Logic ──────────────────────────────────────────────────────
 
-func rollAdvTreasureDrop(tier int, userID id.UserID) *AdvTreasureDrop {
+func rollAdvTreasureDrop(tier int, userID id.UserID, chatLevel int) *AdvTreasureDrop {
 	rate, ok := advTreasureDropRates[tier]
 	if !ok {
 		return nil
 	}
+	rate += chatLevelRareBonus(chatLevel)
 
 	if rand.Float64() >= rate {
 		return nil