everruns-core 0.18 becomes a neutral execution kernel

In 0.18, everruns-core keeps only the execution kernel. Persisted records and hosted services (sessions, agents and agent versions, harnesses, evals and observers, connectors, system email, feature flags, OAuth and credential infrastructure) moved to everruns-platform, everruns-host, everruns-provider and other owning crates. The logic-free everruns-runtime compatibility crate is removed, and engines now own the session lifecycle on one execution path for in-process and durable runs.

REST and gRPC shapes and the stored schema are unchanged; code that imports from everruns_core needs new paths. Self-hosted servers need no change.

See Upgrade notes: 0.18, #3160, #3101 and #3179.

Update 2026-10-04

Several crates named here were later consolidated: everruns-platform became everruns-capabilities in 0.39, and everruns-host became an everruns-core feature in 0.40. See Framework crates consolidated into everruns-core and the facade.