Automate manifesto signing: form → mailing list + auto-published signatory list

Currently manifesto signing is a Google Form processed manually (signatories hand-added to manifesto.md).

Desired:
1. On signup, signatory is added to the mailing list (Substack or ESP — pending email-infra decision, see devspaces-consolidation/BRIEF.md).
2. Signatory list on /manifesto updates automatically (with consent + affiliation-disclaimer handling).

Needs its own mini research/plan:
- Options: Google Form + Apps Script → commit to repo / Brevo form + API → build-time fetch / Tally+webhook / custom endpoint.
- Consent language for public listing; moderation step to prevent spam signatures.
- Where the canonical signatory store lives (sheet vs repo data file).

Part of the Developmental Spaces consolidation (see life-itself/devspaces-consolidation planning repo: BRIEF.md, PLAN.md).

Start by reading BRIEF.md and PLAN.md in the life-itself/devspaces-consolidation planning repository, then review the listed form, mailing-list, webhook, and build-time options. Done means documenting an agreed architecture, canonical signatory store, consent and affiliation-disclaimer language, moderation step, and implementation plan for automatic updates.

Written by the indexing model from the issue text.