Use a reasonable version value for `chaingraphUserAgent`

[bitjson]

Right now the agent pulls the version from package.json, which isn't necessarily a useful version value:

https://github.com/bitauth/chaingraph/blob/ad0797a2b8399ef01244e2afbdf09d66113f3e12/src/config.ts#L12

https://github.com/bitauth/chaingraph/blob/ad0797a2b8399ef01244e2afbdf09d66113f3e12/src/config.ts#L307-L314

It would be better to use the docker image tag for the agent (either the git commit short digest for official images or dev-build for locally-built images or local development agent instances which are not running in a docker image).