One pinned engine version

AgentFS is vendored and its engine patched so the binary links exactly one engine.

Context

Decision 0002 fixes one engine everywhere. In practice the embeddable AgentFS SDK pins its own engine version, and the hub store needs a newer one for native full-text search. Depending on both links two copies of the engine into the binary, which breaks the one-engine and lean invariants and roughly doubles the storage surface.

Decision

Vendor the AgentFS SDK and patch its engine dependency to the single engine version the hub pins. The dependency tree must contain exactly one engine version. The upstream API drift from the pinned version to the current one is sized before any product code is wired.

Consequences

• hub.db keeps native full-text search, and session files run the same engine.
• A dependency that reintroduces a second engine version fails the build.
• The engine is a pre-release, so the version is pinned exactly, never auto-upgraded, and any bump is tested for concurrent writes before it lands.
• Vendoring carries a maintenance duty: when the upstream SDK moves, the patch is reapplied and the drift reassessed.