FR: Session usage tracking

[Lenbok]

I just update my opencode and found that I'm now getting some extra uninterpreted json being inserted into the shell buffer. It corresponds to a feature they've recently implemented to allow tracking session and context usage:

https://agentclientprotocol.com/rfds/session-usage

Here's the opencode PR, where you can see a screenshot of how it renders in an alternate ACP client:

anomalyco/opencode#12299

Is support for that something you'd be interested in having in agent-shell? Presumably we could display the session usage status in the header somewhere.

Small example traffic file including the usage_update notification:
agent-shell-opencode-usage-traffic.txt

[xenodium]

Is support for that something you'd be interested in having in agent-shell?

Certainly. Interested in sending a PR?

We can potentially add (cons :usage nil) to agent-shell--make-state to store all usage details as they come. We can subsequently display wherever we find useful (we may have to play around with this). Could be the header, but who knows, it's getting a little crowded there. Could also be that we output usage into the shell itself when agent turn ends.

Maybe something like:

╭─ Usage ──────────────────────────────────────────────────────────╮
│ Tokens  11,602 total     Context 2 / 200,000     Cost    $0.0439 │
╰──────────────────────────────────────────────────────────────────╯

[Lenbok]

Yep, I'll take a look at it.

[Lenbok]

I think this is fine to close now #270 is merged.