Function cleanArticleTitle

Scrub boilerplate from the raw H1 of an executive-brief so it can be used as the article <title>. Per seo-metadata-contract.md §2:

• strip a leading Executive Brief — / Executive Brief - prefix (the template boilerplate that masks the story)
• when lang is provided, also strip the translated boilerplate prefix in that language (Exekutiv sammanfattning — in SV, Zusammenfassung — in DE, Synthèse exécutive — in FR, etc.). See ./brief-prefixes.ts for the per-language dictionary.
• strip a trailing — YYYY-MM-DD / - YYYY-MM-DD / YYYY-MM-DD (dates belong in article:published_time, not the SERP title)
• if the cleaned title is < 20 chars — too short to be a real story headline — return null so the caller can fall back to a BLUF sentence or to the fallback subfolder-based title.
• if the cleaned title equals (case-insensitive, punctuation-stripped) the prettified subfolder fallback (e.g. Government Propositions for the propositions subfolder), return null so the caller falls back to a BLUF-synthesised title with a real actor + verb. Without this guard, lazy template H1s like # Executive Brief — Government Propositions 2026-05-08 ship as the bare category label.