feat(context): multi-user vault visibility policy

[Svagtlys]

Description

With 2+ humans in one session, vector-search scoping of vault_items.user_id risks cross-user context injection: whose vault items are eligible for a session's assembled context? Record as a known constraint now; design when multi-user sessions enter scope.

Why

The shipped schema scopes vault items to a single user_id, which is unambiguous for the 1-user + 1-agent shape. Multi-human sessions make the injection policy ambiguous — a wrong default leaks one user's vault contents into another user's context.

Scope

• Design decision (ADR) for vault visibility in multi-user sessions
• Injection-time filter policy in the Context Manager when built
• Tests once a multi-user path exists

Not blocked; design when multi-user enters scope. Spec: .agents/specs/2026-09-13-vector-db-schema-orm-design.md (Follow-up issues, item 2).