Fix local D1 demo fixture setup

[sarthakagrawal927]

Summary

• add a local-only Wrangler config pinned to the current D1 migration directory
• create the missing Better Auth base tables and remove the invalid agentAuthCodes.userId index
• add a guarded, repeatable local migration + demo seed command
• align current setup and data documentation with the D1-only runtime

Validation

• pnpm exec vitest run tests/local-d1-setup.unit.test.mjs
• pnpm docs:check (0 errors; 8 pre-existing archive warnings)
• pnpm db:setup:local -- --persist-to /tmp/karte-d1-validation.aaAITa (run twice)
• local query: 11 migrations, 4 demo profiles, dmMode present
• git diff --check

No remote D1 or production configuration was changed.

Closes #36