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-runtime/src/acp/stream.rs

@@ -7,6 +7,8 @@ use std::collections::HashMap;
 use hive_claude::{Telemetry, TokenUsage};
 use serde_json::{Map, Value, json};
 
+use super::ReportedCost;
+
 /// Turns one prompt's `session/update` notifications into claude
 /// `stream-json` events.
 ///
@@ -258,6 +260,19 @@ pub(super) fn config_options(value: &Value) -> Option<Vec<ConfigOption>> {
     )
 }
 
+/// The running total cost a `usage_update` reports for its session, or
+/// `None` for any other `update` and for one that reports no cost.
+pub(super) fn reported_cost(update: &Value) -> Option<ReportedCost> {
+    if update.get("sessionUpdate").and_then(Value::as_str) != Some("usage_update") {
+        return None;
+    }
+    let cost = update.get("cost")?;
+    Some(ReportedCost {
+        amount: cost.get("amount")?.as_f64()?,
+        currency: cost.get("currency")?.as_str()?.to_owned(),
+    })
+}
+
 /// The options a `config_option_update` sets, or `None` for any other
 /// `update`. Each replaces the session's previous options.
 pub(super) fn updated_config_options(update: &Value) -> Option<Vec<ConfigOption>> {
@@ -402,7 +417,8 @@ fn tool_result(id: &str, output: &str, is_error: bool) -> Value {
 #[cfg(test)]
 mod tests {
     use super::{
-        ConfigOption, StreamMapper, canonical_tool_name, config_options, mcp_servers, session_model,
+        ConfigOption, ReportedCost, StreamMapper, canonical_tool_name, config_options, mcp_servers,
+        reported_cost, session_model,
     };
     use serde_json::{Value, json};
 
@@ -537,6 +553,33 @@ mod tests {
         assert_eq!(t.model.as_deref(), Some("provider/model"));
     }
 
+    #[test]
+    fn a_usage_update_reports_its_cost_when_it_carries_one() {
+        assert_eq!(
+            reported_cost(&json!({ "sessionUpdate": "usage_update", "used": 5000,
+                "size": 262_144, "cost": { "amount": 0.045, "currency": "USD" } })),
+            Some(ReportedCost {
+                amount: 0.045,
+                currency: "USD".into()
+            })
+        );
+        assert_eq!(
+            reported_cost(&json!({ "sessionUpdate": "usage_update", "used": 5000,
+                "size": 262_144 })),
+            None
+        );
+        assert_eq!(
+            reported_cost(&json!({ "sessionUpdate": "usage_update",
+                "cost": { "amount": 0.045 } })),
+            None
+        );
+        assert_eq!(
+            reported_cost(&json!({ "sessionUpdate": "plan",
+                "cost": { "amount": 0.045, "currency": "USD" } })),
+            None
+        );
+    }
+
     #[test]
     fn updates_with_nothing_to_show_emit_nothing() {
         let out = feed(