chore(hygiene): untrack BEFUNDE.md and add internal files to .gitignore (T-20260926-434768981)

[lukisch]

Summary

• Untracks internal maintenance file \BEFUNDE.md\ from git (\git rm --cached), preserving the local file.
• Adds \BEFUNDE.md, \MARKETING-LOG.txt, \TASKPLAN_STATUS_*.md, and _after-care/\ to .gitignore.
• Adds regression contract test \ est_gitignore_internal_file_hygiene\ in \ ests/test_metadata.py.
• Documents changes in \CHANGELOG.md\ under ## [Unreleased].

Verification

• 0 references to BEFUNDE in codebase.
• Tests passed: 10/10 in test_metadata.py.
• 
  uff check .\ clean.
• Reference: T-20260926-434768981 (Repo-Hygiene Folgewellen).

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