Default generate to PGlite

[tanchaowen84]

@algora-pbc /claim #2

Implementation notes

• Make pgstrap generate default to the existing PGlite generation path, so generated db:generate scripts no longer require a background Postgres server.
• Keep external Postgres generation available through --no-pglite.
• Restore DATABASE_URL in a finally block and close both the temporary pg-gateway server and PGlite database after generation.
• Update README guidance for the new default and external Postgres opt-out.
• Add regression coverage for default PGlite generation with an invalid external DATABASE_URL, plus DATABASE_URL restoration when generation fails after the temporary PGlite connection is created.

Verification

• bun test - 4 pass, 0 fail, 11 expect() calls
• bun run build - CJS and DTS builds succeeded
• bun run format:check - Checked 15 files, no fixes needed
• git diff --check - passed with no output

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