chore: Upgrade db to use new modusGraph [DRAFT]

[matthewmcneely]

Description

This is a DRAFT PR.

This is a first cut at upgrading the db folder to use the new modusGraph API. At the moment, I've only reworked the AgentState class to serve as a point of discussion.

Discussion Points:

1. Will we be moving away from the Neon/Postgres instances in production? If so, I can refactor even further.
2. I've made the modusGraph::Client the single access point to interacting with modusGraph regardless of whether you're working with a file://-based local db or a dgraph://-based cluster. It might be nice to refactor all the "initdb" stuff to just keep a handle to the Client and operate without all the if useModusDB() tests everywhere (if Postgres will be removed).
3. Another improvement that might be nice is to add lifecycle-functions to the stateful classes themselves, e.g.,

func (a *AgentState) Upsert(ctx context.Context) error {
	client, err := GetClient()
	if err != nil {
		return err
	}
	return client.Upsert(ctx, a)
}

The db/agentstate_test.go file has a lifecycle test of AgentState using the new mg API. Another test file, modusdb_test.go has some of the higher level API functions covered.

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