Syd OS v1: initial fork - rebrand, PH legal context, schema reservations

Rebrand the Mike OSS upstream to "Syd OS" for RX Ventures' internal legal
deployment. Changes preserve upstream LICENSE (AGPL-3.0) and add NOTICE.md
crediting the original project.

- chatTools.ts SYSTEM_PROMPT rewritten as "You are Syd" with the Philippine
  legal context block from the Syd setup spec §7.2 (PH regulators, contract
  types, drafting defaults, citation rules, prohibitions).
- Frontend brand swap across layout metadata, login, signup, sidebar,
  logo, global error page, workflow list badge, support page, and analysis
  prompt placeholders.
- layout.tsx: title/OG/twitter now Syd OS at legal.rxventures.co;
  added robots: noindex/nofollow.
- signup/page.tsx: replaced demo-service notice with "self-signup disabled"
  text (Supabase signups are off in the Syd OS deployment).
- schema.sql: added metadata jsonb columns on documents and workflows
  (default {"source_system":"syd_os"}). Reserved per Syd spec §8 for v1.1
  Capital integration (cap_event_id binding).
- New stub: backend/src/routes/integrations/capital.ts mounts at
  /api/integrations/capital/webhook and returns 501. Reserves the URL path
  for the v1.1 Rx Capital wire-up.

Co-Authored-By: Claude <noreply@anthropic.com>