docs: replace ASCII umlaut fallbacks with proper äöüß across all .md

Most German docs were written with `ue`/`ae`/`oe`/`ss` ASCII fallbacks
(`fuer`, `aenderung`, `koennen`, `gemaess`) — relic from older toolchain
constraints. Replaced 237 occurrences across 38 .md files with proper
umlauts and ß so the docs read naturally.

Substitutions are constrained to plain prose: fenced code blocks
(```...```), inline code spans (`...`), markdown link targets `[..](..)`,
and HTML comments are preserved verbatim. Path references like
`docs/howto-erste-aenderung.md` keep their original (umlaut-replaced)
filenames — only the surrounding prose is normalised.

Tool: bin/fix-md-umlauts.py (idempotent — no-op on already-clean files).
Re-runnable if ASCII fallbacks creep back in via copy-paste.

Doc-link-check + doc-drift-check stay green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.agents/skills/starterkit-planner/references/tradeoff-matrix.md

@@ -19,7 +19,7 @@
 ### Option B: Shared Partial/Helper
 
 - Vorteile: Schneller Nutzen, moderates Risiko
-- Nachteile: Teilweise Restduplikate moeglich
+- Nachteile: Teilweise Restduplikate möglich
 - Geeignet wenn: Wiederkehrendes UI-/Glue-Muster mit klaren Datencontracts
 
 ### Option C: Lokal belassen