Syd OS v1: surface PH workflows as built-ins; fix dead SQL seed

The five PH workflows (spec §7.3) were seeded as is_system=true rows in
backend/seed/01_workflows.sql, but nothing in the upstream Mike app reads
is_system=true - the workflow list route returns only own+shared rows, and
the chat workflow store reads the hardcoded builtin arrays. The seeded rows
were therefore invisible, while the generic upstream built-ins (CP checklist,
credit agreement, LPA, England & Wales lease, e-discovery, etc.) still showed.

- Replace both built-in registries with the five PH workflows, matching IDs:
    backend/src/lib/builtinWorkflows.ts (chat-assistant tool store)
    frontend/src/app/components/workflows/builtinWorkflows.ts (Built-in tab)
  Removes the off-jurisdiction / off-domain generic workflows. The tabular
  review feature stays available for user-created workflows; PH tabular
  templates can be added later.
- Mark backend/seed/01_workflows.sql SUPERSEDED (running it would insert
  invisible duplicate rows) and update seed/README.md accordingly.
- README.md: rebrand header Mike -> Syd OS with upstream credit; note that
  production signups are disabled (email+password behind Cloudflare Access).

Both apps build clean (tsc; next build with Supabase env set).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>