Default db:generate to PGlite

[swhan0329]

@algora-pbc /claim #2

Closes #2

Summary

• makes pgstrap generate default to the existing PGlite path so bun run db:generate and direct generate() calls no longer need a local Postgres server
• keeps external Postgres generation available via --no-pglite
• restores DATABASE_URL and closes the temporary PGlite gateway/database in a finally block
• updates README guidance and regression coverage for the default PGlite path

Verification

• bun test tests/generate.pglite.test.ts tests/init.test.ts
• bun test
• bun run build
• bun run format:check
• git diff --check

[swhan0329]

/claim #2

[swhan0329]

@algora-pbc /claim #2

[swhan0329]

Fresh local verification because this PR does not have GitHub checks attached:

• bun test tests/generate.pglite.test.ts tests/init.test.ts -> 2 tests passed
• bun test -> 3 tests passed
• bun run format:check -> passed after tests completed and removed their temporary migration fixture
• bun run build -> passed
• git diff --check -> passed

The change remains scoped to making pgstrap generate/scaffolded db:generate use PGlite by default, with --no-pglite preserving the external Postgres opt-out.