fix: default db:generate to PGlite so Postgres is not required

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@algora-pbc /claim #2

Fixes #2 — db:generate no longer needs a Postgres server running.

generate already had a working PGlite path, it was just off by default. This PR flips the default instead of adding new machinery:

• src/cli.ts: --pglite now defaults to true, so yargs gives you --no-pglite as the escape hatch for generating against a real database. The handler now awaits generate(), which it previously did not — without that the CLI could exit before generation finished.
• src/generate.ts: pglite defaults to true so programmatic generate(ctx) behaves like the CLI, and the PGlite TCP server is closed (and DATABASE_URL restored) in a finally block so a failed generation cannot leak a listening socket.
• src/init.ts: scaffolds pgstrap generate --pglite — explicit intent in new projects even though the CLI default already covers it.
• README: one line saying npm run db:generate uses in-memory PGlite and needs no Postgres, plus --no-pglite for an existing database.
• Tests: init.test.ts updated; a short test in the existing generate.pglite.test.ts asserts the default path generates types with no server running.

Why this over the other open PRs: changing only the init script leaves every already-initialized project still running pgstrap generate against a Postgres that isn't there, so the issue isn't actually fixed for them. Adding a separate db:generate:postgres script invents surface the issue didn't ask for, and the larger rewrites (env sniffing, restructured generate.ts, lockfile churn) change far more than the bug requires. This is 5 files, no new dependencies, no reformatting noise. bun test and bun run format:check are green.