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_masterwork.go

@@ -169,8 +169,9 @@ func (p *AdventurePlugin) checkMasterworkDrop(userID id.UserID, char *AdventureC
 		return // no masterwork available for this activity+tier (e.g. dungeon)
 	}
 
-	// Roll for drop
-	if rand.Float64() >= def.DropRate {
+	// Roll for drop (chat level rare bonus applied additively)
+	dropRate := def.DropRate + chatLevelRareBonus(p.chatLevel(userID))
+	if rand.Float64() >= dropRate {
 		return
 	}