N5 review fixes: protect keys from Robbie, fire epilogue on autopilot, atomic finale reward

Five correctness fixes from a code review of the N5 branch:

- Robbie no longer sweeps/sells cross-zone keys (Type "key"), which
  permanently broke the vault unlock they exist to open.
- Robbie's gift tier now reads the canonical DnD level, not the frozen
  legacy CombatLevel that pegged every gift at tier 1.
- Boss epilogue (D1b) now fires on the compact autopilot boss resolve —
  the primary long-expedition path — not just manual !fight. Deduped the
  two manual sites into a shared writeBossEpilogue helper.
- Finale reward latches epilogue_cleared before granting the Legendary +
  title, so a failed/late write can't make the reward repeatable.
- Misty arc beat's occupied-slot guard moved above the counter increment,
  so a contended pending slot defers the encounter instead of consuming a
  5/15/30 beat forever.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/adventure_npcs.go

@@ -131,6 +131,15 @@ func (p *AdventurePlugin) npcFireEncounter(userID id.UserID, npc string) {
 		return
 	}
 
+	// Don't consume the encounter — or its one-shot arc beat — if the player is
+	// already mid-interaction (shop, treasure, another NPC). Bail before
+	// touching MistyEncounterCount so a contended slot defers the whole
+	// encounter to a later fire instead of durably advancing the counter past
+	// a 5/15/30 threshold and losing that beat forever.
+	if _, occupied := p.pending.Load(string(userID)); occupied {
+		return
+	}
+
 	now := time.Now().UTC()
 
 	var opening, prompt string
@@ -165,11 +174,6 @@ func (p *AdventurePlugin) npcFireEncounter(userID id.UserID, npc string) {
 		slog.Error("player_meta: npc last_seen dual-write failed", "user", userID, "npc", npc, "err", err)
 	}
 
-	// Don't overwrite an existing pending interaction (shop, treasure, etc.)
-	if _, occupied := p.pending.Load(string(userID)); occupied {
-		return
-	}
-
 	// Set pending interaction — NPC encounters stay valid until end of UTC day
 	endOfDay := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, time.UTC).Add(24 * time.Hour)
 	p.pending.Store(string(userID), &advPendingInteraction{