hive-agent: export ACP-reported cost and context fill over OTLP

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

hive-agent/src/main.rs

@@ -1001,6 +1001,7 @@ async fn handle_turn<S: Surface>(
     let ended_at = chrono::Utc::now().timestamp();
     let duration_ms = i64::try_from(started_instant.elapsed().as_millis()).unwrap_or(i64::MAX);
     let (open_threads, open_reminders) = S::post_turn_counts(socket).await;
+    let context_window = bus.effective_context_window(&model_at_start);
     let row = serve_common::build_row(serve_common::TurnRowArgs {
         started_at,
         ended_at,
@@ -1014,10 +1015,17 @@ async fn handle_turn<S: Surface>(
         open_reminders_count: open_reminders,
     });
     // Harness-only OTEL metrics (duration/wake_from/result_kind/loose-ends/
-    // session boundaries) — independent of the sqlite sink below, and a
-    // cheap no-op when OTEL isn't configured. See `otel_turn_metrics`'s
-    // module doc for why token/cost/tool-count are deliberately not here.
+    // session boundaries/context fill) — independent of the sqlite sink
+    // below, and a cheap no-op when OTEL isn't configured. See
+    // `otel_turn_metrics`'s module doc for why token/tool-count are
+    // deliberately not here, and why cost is only ever an ACP agent's.
     otel_turn_metrics::record(&row, fresh_session);
+    if let Some(ctx) = bus.last_ctx_usage() {
+        otel_turn_metrics::record_context(ctx.context_tokens(), context_window);
+    }
+    for cost in hive_runtime::Runtime::take_reported_cost(session) {
+        otel_turn_metrics::record_reported_cost(&row.model, &cost);
+    }
     if let Some(stats) = stats {
         stats.record(&row);
     }