fix(core): runtime-neutral legacy credential import

[kitlangton]

What

Bun.file crashes at database bootstrap in plain Node and Cloudflare workerd because Bun is undefined, preventing fresh databases from completing migrations.

How

• Read legacy auth.json with node:fs/promises readFile.
• Skip import silently when the file is absent or any read fails.
• Preserve malformed-content failures and the existing importLegacyCredentials(tx, filepath) signature.

Testing

• bunx tsgo --noEmit in packages/core
• bun test test/database-migration.test.ts in packages/core (9 passed)
• Added coverage verifying an absent legacy credential file succeeds without importing rows.