I'm pretty happy using Shelley, which stores agent conversations in a Sqlite database. I can refer to a previous conversation and the agent can easily do a Sqlite query to see what happened.

Although this isn't stored in git, I don't see any particularly need to since it's too detailed. Instead I have the agent write design docs (as an alternative to plan mode) and check those in. That seems like enough.

Doesnt the tooling already exist? i.e. you could use `git notes` to attach agent state as checkpoints to Trees, Commits, Tags etc.

The CLI is open source, everyone can use it and it does work with git only. So, no separate platform needed. The platform only provides convenience to view checkpoints at the moment. However you can also view them in the CLI. It's here https://github.com/entireio/cli