Add the channel messages and the usage record to proto

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/proto/src/log.rs

@@ -12,6 +12,17 @@ pub struct ToolCall {
     pub arguments: String,
 }
 
+/// What one completion cost. The same five numbers as `LogRecord::Usage`, without the time.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct Usage {
+    pub cache_n: u64,
+    pub prompt_n: u64,
+    pub predicted_n: u64,
+    pub reasoning_tokens: u64,
+    pub thinking_capped: bool,
+}
+
 // JSON: {"type":"user","time":"…","content":"…"} ; the tag sits beside the fields; types are snake_case
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(tag = "type", rename_all = "snake_case", deny_unknown_fields)]
@@ -42,6 +53,14 @@ pub enum LogRecord {
         untrusted: bool,
         truncated: bool,
     },
+    Usage {
+        time: Timestamp,
+        cache_n: u64,
+        prompt_n: u64,
+        predicted_n: u64,
+        reasoning_tokens: u64,
+        thinking_capped: bool,
+    },
     CacheLoss {
         time: Timestamp,
         expected: u64,