docs: remove Briefwechsel from architecture, routes and glossary

Drop the Briefwechsel route and the conversation derived-domain /
conversation-thread prose from the route tables (CLAUDE.md,
frontend/CLAUDE.md), ARCHITECTURE.md, the C4 frontend/backend diagrams,
and GLOSSARY.md (term + derived-domain list). Delete the two superseded
Briefwechsel design specs. Historical ADRs and dated analyses are left
untouched as point-in-time context.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -194,7 +194,6 @@ frontend/src/routes/
 │   ├── [id]/edit/          Person edit form
 │   ├── new/                Create person form
 │   └── review/             Triage view — confirm/rename/merge/delete provisional persons
-├── briefwechsel/           Bilateral conversation timeline (Briefwechsel)
 ├── aktivitaeten/           Unified activity feed (Chronik)
 ├── geschichten/            Stories — list, [id], [id]/edit, new
 ├── stammbaum/              Family tree (Stammbaum)