Save chains as files we can version and sync

Chains live in hangar.db on one machine. I keep the rest of my pipeline definitions in a git repository and would like Hangar's chains to sit beside them. A good outcome is export and import of a chain as a JSON or YAML file, plus an optional chains folder that Hangar watches so a chain placed there appears in the app. Combined with a command-line runner, this gives one source of truth for a chain that I can run by hand in Hangar, from a script, or from an agent.