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/lib.rs

@@ -23,7 +23,7 @@ mod spec;
 
 pub use acp::{
     AcpError, AcpRuntime, Canceller, Choice, Choices, PermissionAsk, PermissionPolicy,
-    SessionChoices,
+    ReportedCost, SessionChoices,
 };
 pub use claude::ClaudeRuntime;
 pub use hive_claude::{
@@ -65,6 +65,11 @@ pub trait Runtime {
     /// pick. `None` for a runtime whose session offers none to read: claude's
     /// `--model` and `--effort` are passed through unchecked.
     fn choices(&self) -> Option<Choices>;
+
+    /// What the agent reported spending since the last call, one entry per
+    /// currency. Empty for a runtime whose agent reports no cost: claude
+    /// exports its own.
+    fn take_reported_cost(&self) -> Vec<ReportedCost>;
 }
 
 /// The runtime an agent was configured with, chosen at startup from a
@@ -109,6 +114,13 @@ impl<P: CompactionPolicy> Runtime for AgentRuntime<P> {
             Self::Acp(r) => r.choices(),
         }
     }
+
+    fn take_reported_cost(&self) -> Vec<ReportedCost> {
+        match self {
+            Self::Claude(r) => r.take_reported_cost(),
+            Self::Acp(r) => r.take_reported_cost(),
+        }
+    }
 }
 
 /// Why a runtime operation did not complete.