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/claude.rs

@@ -4,7 +4,7 @@ use std::path::PathBuf;
 
 use hive_claude::{CompactionPolicy, Config, InfiniteSession, Progress, SessionStore, Sink};
 
-use crate::{Canceller, Choices, Result, Runtime};
+use crate::{Canceller, Choices, ReportedCost, Result, Runtime};
 
 /// `claude --print` turns on a titled [`InfiniteSession`], which owns
 /// resume-or-create and compaction.
@@ -47,4 +47,8 @@ impl<P: CompactionPolicy> Runtime for ClaudeRuntime<P> {
     fn choices(&self) -> Option<Choices> {
         None
     }
+
+    fn take_reported_cost(&self) -> Vec<ReportedCost> {
+        Vec::new()
+    }
 }