fix: default generate to pglite

[vahapogut]

Fixes #2

@algora-pbc /claim #2

Summary

• Make pgstrap generate use PGlite by default so generated db:generate works without a local Postgres server.
• Keep external Postgres available through pgstrap generate --no-pglite.
• Restore DATABASE_URL and close the temporary PGlite gateway/database in finally.
• Update init scaffolding, README guidance, and regression coverage.

Verification

• bun test
• bun run build
• bun run format:check
• git diff --check