Add first-class X social lead intake

[rightclaw]

Summary

• add a bounded first-class X social lead intake contract and file-backed store
• expose social lead intake and read endpoints plus /api/data discovery surface
• record Epic 72 task packets and dev cycle 191 closeout artifacts

Validation

• node --test test/social-lead-store.test.mjs test/social-leads-endpoint.test.mjs test/local-fork-planning-hygiene.test.mjs
• npm run restart:safe
• verified /api/health, /api/data, and /api/social-leads on the runtime