refactor(opencode): drop AppRuntime from instance-runtime

Builds a local ManagedRuntime over InstanceLayer.layer with the shared
process-wide memoMap, so the four exported Promise helpers (load,
disposeInstance, disposeAllInstances, reloadInstance) no longer go
through AppRuntime. Service identity is preserved via memoMap; every
caller's behavior is unchanged.

One step in the broader AppRuntime removal. All public-facing exports
stay the same — callers in src/cli/bootstrap, src/cli/cmd/tui/worker,
src/acp/runtime, and the test fixtures don't need to change.