Add a telos usage command for token/cost visibility

[akshayakula]

Hey! Was good chatting yesterday. We're trying Telos out for our deploy agent's background work — running pi with Cerebras GLM 4.7 as the model — and the first thing we reached for was a quick way to see token burn across sessions. list has the COST column, but with a custom provider the pricing comes back zero, so the raw token counts are what actually matter for us.

This adds a small, purely additive telos usage command:

$ telos usage
NAME           STATUS      INPUT  OUTPUT  CACHE-R  CACHE-W  COST     SESSION
hello-service  completed   1200   300     5000     800      $0.2500  sess_a
TOTAL          1 sessions  1200   300     5000     800      $0.2500

• Reads the same stores as list (root session tree when delegated, local store otherwise); child sessions stay in the report so totals don't undercount
• --json emits rows plus aggregated totals (we feed this into our usage dashboard)
• Sessions that haven't reported stats yet render -

gofmt clean, go build ./... and go test ./... pass locally.

Happy to rename/reshape (e.g. fold it into list --usage) if you'd prefer. Also noticed usage totals only land in the store at game_end, so running sessions show nothing — happy to take a swing at live running totals as a follow-up if that'd be welcome.

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