Weekly digest to Discord #29

Why

A short weekly summary keeps the community and the maintainers looking at the data.

What to build

• A digest subcommand that compares the last two weekly runs (survival points, graduations, SCF links, wherever those tables exist) and renders a short markdown summary. Sections whose tables do not exist yet are skipped.
• It posts to a Discord webhook if DISCORD_WEBHOOK_URL is set, and prints otherwise.
• Wire it into the survival workflow after the survival step.

Acceptance criteria

• A fixture of two runs produces a deterministic digest.

See CONTRIBUTING.md for conventions.