Make generated db:generate use PGlite by default

[CyrusAuyeung]

Summary

• Make pgstrap init scaffold db:generate as pgstrap generate --pglite, so generated projects can run bun run db:generate without a local Postgres server.
• Add db:generate:postgres as an explicit script for the original real-Postgres generation path.
• Update the init test and README script docs.

Why this shape

@seveibar I noticed a few existing attempts for this issue. This keeps the CLI default unchanged to avoid surprising existing pgstrap generate users, while making newly initialized projects satisfy #2 out of the box. It also preserves the Postgres path with a named script instead of changing global generate semantics.

If this smaller approach is the direction you prefer and it gets merged, could this be considered for the Algora bounty on #2?

Verification

• npm exec --yes bun -- test
  • 3 pass, 0 fail
  • Windows emitted repeated The system cannot find the path specified. messages from the migration tooling, but the test process exited successfully.
• npm exec --yes bun -- run build
• npm exec --yes bun -- x biome format README.md src/init.ts tests/init.test.ts
• git diff --check

Fixes #2

@algora-pbc /claim #2