fix: complete extension usage accounting

closes #6509

packages/coding-agent/docs/session-format.md

@@ -96,6 +96,7 @@ interface ToolResultMessage {
   toolName: string;
   content: (TextContent | ImageContent)[];
   details?: any;      // Tool-specific metadata
+  usage?: Usage;      // Nested LLM work performed by the tool
   isError: boolean;
   timestamp: number;
 }
@@ -232,6 +233,7 @@ Created when context is compacted. Stores a summary of earlier messages.
 ```
 
 Optional fields:
+- `usage`: LLM usage from generating the summary; included in session token and cost totals
 - `details`: Implementation-specific data (e.g., `{ readFiles: string[], modifiedFiles: string[] }` for default, or custom data for extensions)
 - `fromHook`: `true` if generated by an extension, `false`/`undefined` if pi-generated (legacy field name)
 
@@ -244,6 +246,7 @@ Created when switching branches via `/tree` with an LLM generated summary of the
 ```
 
 Optional fields:
+- `usage`: LLM usage from generating the summary; included in session token and cost totals
 - `details`: File tracking data (`{ readFiles: string[], modifiedFiles: string[] }`) for default, or custom data for extensions
 - `fromHook`: `true` if generated by an extension, `false`/`undefined` if pi-generated (legacy field name)