docs: synchronize German README parity

[lukisch]

Outcome

Synchronizes the maintained German README with the canonical English onboarding structure without changing runtime behavior.

Scope

• add the missing Start Here quick-reference section
• align banner and onboarding hierarchy
• preserve all eight code and Mermaid blocks raw-byte-identically
• correct one German note sentence
• record the synchronization in CHANGELOG.md

Verification

• 15/15 heading levels aligned
• 8/8 fenced blocks raw-byte identical
• 14/14 links; localized language switch, badges and anchors retained
• 61 tests passed plus 15 subtests
• compileall and git diff --check passed
• independent review: APPROVE, no P0/P1/P2

Workflow context

The unrelated first-interaction v3 input bug observed on the earlier PR was fixed separately and merged in PR #2.

Nonclaims

No runtime, memory-store, publication, authority, or broader ecosystem language-parity change.

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