An ACP agent's `usage_update` carries `cost.{amount,currency}`, the
session's running total (opencode sums every assistant message in the
session). hive-runtime now reads it and turns the running total into
what each report added: a new session counts from zero, a session loaded
into a freshly started agent only baselines on its first report, and a
falling total adds nothing. The spend is held on the runtime until
`Runtime::take_reported_cost` drains it; claude's runtime reports none,
since the claude binary already exports `claude_code.cost.usage`.
hive-agent's existing turn-metrics meter records three new instruments:
- `hyperhive.agent.cost.usage` (counter, `model` + `currency`), ACP only;
- `hyperhive.agent.context.used` / `.size` (gauges, no attributes), for
every backend: the two numbers the web UI's ctx% divides.
The `hyperhive · agents` dashboard gets ACP cost panels on its cost tab
and a context-fill panel on its health tab.
Refs #4845
c2bdf30e05