docs(adr): ADR-0020 for the acknowledgement mechanism

The framework audit found this missing. Acknowledging known findings is a process
decision about how the alarm system treats exceptions, and AGENTS.md is explicit
that documenting an exception instead of deciding it is itself the error. It lived
only inside #0104 and #0105, which are issues, not decision records.

The ADR carries the reasoning the issues could not: why option C beat working the
backlog down first or tolerating red, and why the obvious objection - an exception
list is a candidate for the next blind spot - is answered by the three rules rather
than waved away. It also records what is deliberately not acknowledged, the
transient mirror divergence, because that message is the only signal if a mirror
truly stops.

One consequence is stated plainly rather than discovered later: acknowledgements
bind to substrings of the finding text, so fixing or moving a cause can change the
wording and require the entry to follow. Stable finding IDs would avoid that and
would make the file unreadable without special knowledge; the trade is taken
knowingly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

STATUS.md

@@ -78,7 +78,7 @@ Verteilung: M1 18 · M2 17 · M3 4 · M4 11 · M5 15
 
 _none active_
 
-## ADRs (19)
+## ADRs (20)
 
 | ADR | Status | Title |
 |---|---|---|
@@ -101,6 +101,7 @@ _none active_
 | [0017](docs/adr/0017-split-dns-cfgmon-vier-zonen.md) | accepted | 0017 — Split-DNS auf CFGMON: vier Zonen statt einer, je Zone begründet |
 | [0018](docs/adr/0018-ki-geraeuschunterdrueckung-clientseitig-opt-in.md) | accepted | 0018 — KI-Geräuschunterdrückung in threadnet-call: client-seitig, opt-in, selbst ausgeliefert |
 | [0019](docs/adr/0019-komponenten-issues-adoptiert.md) | accepted | ADR-0019: Komponenten-Issues in docs/issues/ adoptiert — eine Nummernwelt für die Gruppe |
+| [0020](docs/adr/0020-bekannte-befunde-quittieren.md) | accepted | ADR-0020: Bekannte Befunde werden quittiert, damit Rot wieder etwas bedeutet |
 
 ## Open AARs (0)