Counters: forget the totals of objects that no longer exist

hc counters kept listing the objects of a configuration replaced since
housecarld started (an import with --replace gives every object a new
ID) beside the current ones. Disabled objects, built-ins and policies
keep their totals.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/operations.md

@@ -154,7 +154,9 @@ table on its hook, so a drop in any of them wins.
 Every rule, NAT rule, built-in and policy has a counter. `hc rule list`,
 `hc forward list` and the web UI show packets; `hc counters` shows them
 all, with bytes. They count from housecarld's start, and carry across
-changes to the ruleset.
+changes to the ruleset. A deleted object's counter goes with it; an
+import with `--replace` gives every object a new ID, so its counters
+start again. A disabled rule keeps its count for when it is enabled.
 
 ## Quotas