core: regression gate for scoped-request isolation (namespace/project leak) #1467

Problem

Issue #1353 (namespace orphans from silent writes) showed one shape of scope isolation failure. The broader class: any search surface returning records from outside its requested scope. This must be a regression gate, not a one-off fix; new search features (hybrid modes, expansion endpoints, MCP parity) keep adding surfaces that can leak.

Proposal

Add a CI-required isolation suite:

• Seed a store with two namespaces (and two project tags) using deliberately overlapping vocabulary.
• Assert zero cross-scope hits on every retrieval surface: recall, FTS search, hybrid/unified recall, room recall, doc search, and ID-based expansion/fetch (fetching a record ID from scope A while scoped to B must respect the filter).
• Include the server (HTTP), CLI, and MCP paths; a surface that exists in one transport must be covered in all.
• Wire as a required CI check: a failure blocks merge. Best-effort tests do not count.

Acceptance criteria

• Suite covers all retrieval surfaces in all three transports
• Marked required in branch protection; failure blocks merge
• Documents which surfaces are covered so new endpoints get added to the suite