docs(db): note user IDs are not always UUIDs

[kilo-code-bot]

Summary

Adds guidance to packages/db/AGENTS.md clarifying that a user ID is not always a UUID.

We keep hitting backwards-incompatible bugs because code assumes user IDs are UUIDs. In reality a user ID is an arbitrary string: it may be a UUID or another format, and OAuth user IDs usually start with the prefix oauth/. This matches the schema, where kilocode_users.id and all user-ID references (e.g. kilo_user_id, user_id) are text() columns, not uuid().

Change

New "User IDs" section under the shared PostgreSQL data-contract notes, warning against UUID validation/parsing and uuid()-typed columns or ::uuid casts on user IDs, since those break for OAuth users.

Docs-only; no code or schema changes.

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