feat: add CNS death notification source record

[HirenGajjar]

Closes #17

Adds data/sources/lu/cns-death-notification.yaml — a source record for the official CNS page on death and funeral costs.

---

Source details

• URL: https://cns.public.lu/en/assure/droits-demarches/dossiers-thematiques/famille/deces-frais-funeraires.html
• Title: Death and funeral costs / Décès et frais funéraires
• Institution: CNS (Caisse nationale de santé)
• Languages: EN, FR (both versions confirmed on cns.public.lu)
• source_type: institutional_page — CNS is a specific institution on .public.lu, distinct from the general Guichet government portal
• verification_status: discovered as required by the issue

Page content

The page covers two areas relevant to bereavement:

1. Funeral allowance — eligibility, claim process, required documents
2. Steps for family members to maintain social security coverage after a death

Notes

• institution_ids is empty — no CNS institution record exists yet in data/institutions/lu/. A separate institution record for CNS should be created to link this source properly.
• URL verified in browser — page loads correctly, title matches.
• Followed guichet-death-life-event.yaml as template per issue guidance.

Verified

• python3 scripts/validate.py — passes
• Trailing newline present
• 1 file created, 1 untracked file staged

[TommiLindfors]

Merged! The CNS funeral allowance is one of the most time-sensitive entitlements families need to know about. This source is now ready for the checklist build.