Remove the setup_banner feature (admin-editable /setup page banner) and
all associated code: API router, repository, renderer, admin template,
tests, and docs. The setup_page handler no longer calls render_setup_banner;
the install.html template no longer renders banner_html. The setup_banner
DuckDB table (v22) is kept intact for forward-compat with already-migrated
instances — only the application code is removed.
CHANGELOG updated: setup_banner bullets removed; Agent Setup Prompt
(welcome-template feature) now stands alone as the single editable prompt.
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