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/dnd_zone_cmd.go

@@ -1130,6 +1130,13 @@ func (p *AdventurePlugin) resolveCombatRoom(userID id.UserID, run *DungeonRun, z
 			ob.WriteString("\n")
 			ob.WriteString(line)
 		}
+		// D1b campaign capstone. The compact autopilot resolves boss rooms
+		// itself (the primary long-expedition path), so the epilogue has to
+		// fire here too — otherwise almost every player clears the boss
+		// without ever seeing it.
+		if isBoss {
+			writeBossEpilogue(&ob, zone.ID)
+		}
 		outcome = ob.String()
 		return
 	}