feat(alerts): a rule can fire when a reading rises, "rose by N" (#6724)

A rule compared a reading with a fixed number, so Synology Disk 1's bad
sectors going from 3,160 to 3,392 could say nothing: a count that only grows
is soon past any line. "rose by" compares each reading with the one before it,
kept on the alert's state and only for such a rule. A fall (a replaced disk)
doesn't fire. On the rule form, through MCP, and worded in emails, the alert
sentence, history and the timeline.

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

steward/mcp/tools/overview.py

@@ -127,7 +127,7 @@ def alert_findings(active, silent: dict[str, list[str]] | None = None) -> list[d
             what += _also_silent(silent.get(rule.resource_name))
         out.append(finding(
             sev, f"alert_{s}", rule.name,
-            f"{rule.metric_name} {rule.operator.value} {rule.threshold:g} on "
+            f"{rule.metric_name} {rule.operator.said} {rule.threshold:g} on "
             f"{rule.source_module}/{rule.resource_name} {what}",
             since=state.no_data_since if s == "no_data" else state.last_transition_at,
             ids={"rule_id": rule.id},