feat(agents): Current branch and time spent in the agent session header

A local-checkout task shows the repository's current branch in its header, and
Changes compares against it; both follow branch switches. The header also shows
the time the agent has spent on the task's turns.

A usage update without a cost (Claude sends one for its rate limits) no longer
clears the session cost.

crates/ide-agent/src/config.rs

@@ -104,17 +104,17 @@ pub(crate) fn commands_of(update: &Value) -> Vec<AgentCommand> {
         .collect()
 }
 
-/// `usage_update`: `{ used, size, cost?: { amount, currency } }`.
-pub(crate) fn usage_of(update: &Value) -> Option<AgentUsage> {
+/// `usage_update`: `{ used, size, cost?: { amount, currency } }`. An update without a cost
+/// keeps the last one (Claude also sends one for its rate limits, without the cost).
+pub(crate) fn usage_of(update: &Value, prev: Option<AgentUsage>) -> Option<AgentUsage> {
     let tokens = |key: &str| update.get(key)?.as_f64().map(|n| n.clamp(0.0, f64::from(u32::MAX)) as u32);
-    let used = tokens("used")?;
-    let size = tokens("size").filter(|s| *s > 0)?;
-    Some(AgentUsage {
-        used,
-        size,
-        cost: update.pointer("/cost/amount").and_then(Value::as_f64),
-        currency: update.pointer("/cost/currency").and_then(Value::as_str).map(str::to_owned),
-    })
+    let (Some(used), Some(size)) = (tokens("used"), tokens("size").filter(|s| *s > 0)) else { return prev };
+    let (cost, currency) = match (update.pointer("/cost/amount").and_then(Value::as_f64), prev) {
+        (Some(cost), _) => (Some(cost), update.pointer("/cost/currency").and_then(Value::as_str).map(str::to_owned)),
+        (None, Some(p)) => (p.cost, p.currency),
+        (None, None) => (None, None),
+    };
+    Some(AgentUsage { used, size, cost, currency })
 }
 
 fn legacy(
@@ -217,8 +217,16 @@ mod tests {
         ]}));
         assert_eq!(cmds[0].hint.as_deref(), Some("what to review"));
         assert_eq!((cmds[1].name.as_str(), cmds[1].hint.as_ref()), ("init", None));
-        let u = usage_of(&json!({"used": 1200, "size": 200000, "cost": {"amount": 0.42, "currency": "USD"}})).unwrap();
+        let u = usage_of(&json!({"used": 1200, "size": 200000, "cost": {"amount": 0.42, "currency": "USD"}}), None)
+            .unwrap();
         assert_eq!((u.used, u.cost, u.currency.as_deref()), (1200, Some(0.42), Some("USD")));
-        assert!(usage_of(&json!({"used": 1, "size": 0})).is_none());
+        assert!(usage_of(&json!({"used": 1, "size": 0}), None).is_none());
+    }
+
+    #[test]
+    fn usage_without_a_cost_keeps_the_last_one() {
+        let u = usage_of(&json!({"used": 100, "size": 1000, "cost": {"amount": 1.5, "currency": "USD"}}), None);
+        let u = usage_of(&json!({"used": 200, "size": 1000, "_meta": {"_claude/rateLimit": {}}}), u).unwrap();
+        assert_eq!((u.used, u.cost, u.currency.as_deref()), (200, Some(1.5), Some("USD")));
     }
 }

crates/ide-agent/src/lib.rs

@@ -915,7 +915,7 @@ impl Task {
                     true
                 }
                 "usage_update" => {
-                    s.task.usage = config::usage_of(update).or(s.task.usage.take());
+                    s.task.usage = config::usage_of(update, s.task.usage.take());
                     true
                 }
                 _ => false,