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/combat_cmd.go

@@ -382,9 +382,7 @@ func (p *AdventurePlugin) finishCombatSession(userID id.UserID, sess *CombatSess
 			b.WriteString(drop + "\n")
 		}
 		if !elite {
-			if ep := bossEpilogueLine(zone.ID); ep != "" {
-				b.WriteString("\n" + ep + "\n")
-			}
+			writeBossEpilogue(&b, zone.ID)
 		}
 		if bossOnExpedition {
 			// The boss is the expedition's climax. Frame the close-out as