RPC clients (the hephaestus.nvim plugin, for its `:Heph version`
command) had no way to learn which hephd they are talking to — `health`
returns counts, not a version. Add a tiny `version` method returning
`{ version: heph_core::VERSION }`, the same `X.Y.Z (sha)` string the
binaries print for --version. No store access needed.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
