Make generated db:generate use PGlite by default

[Veape229]

Closes #2
/claim #2

Summary

• Update initPgstrap so newly initialized projects run pgstrap generate --pglite for db:generate.
• Update the init test expectation to match the generated script.
• Clarify in the README that npm run db:generate uses an in-memory PGlite database to generate types and structure from migrations.

Testing

• npx biome check README.md src/init.ts tests/init.test.ts
• npm run build with a local typescript@5.4.5 install
• Verified initPgstrap writes db:generate as pgstrap generate --pglite in a temporary project

Note: I could not run bun test locally because Bun is not installed in this environment.