Project-scoped recall plus an opt-in user memory tier

Memory recall is project-scoped, plus a new explicit, opt-in user-level memory tier across projects. Every part of it is configurable and exposed as settings, signals and actions: enable, scope, what is promoted, retention, visibility and editing, recall weight.

Carried by MEM-8 (was RC-48) since MEM-DIR, in the per-user shard of U-M1.