feat(agent): support custom metadata in jsonl session headers (#6417)

Allow callers to attach an opaque JSON object to the session header so
application context needed to rebuild a harness (for example an agent
profile reference) is readable from the first line alone, without
scanning session entries. The field is optional and ignored by readers
that do not use it; fork inherits the source metadata unless overridden.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/agent/src/harness/session/jsonl-repo.ts

@@ -85,6 +85,7 @@ export class JsonlSessionRepo implements JsonlSessionRepoApi {
 			cwd: options.cwd,
 			sessionId: id,
 			parentSessionPath: options.parentSessionPath,
+			metadata: options.metadata,
 		});
 		return toSession(storage);
 	}
@@ -150,6 +151,7 @@ export class JsonlSessionRepo implements JsonlSessionRepoApi {
 				cwd: options.cwd,
 				sessionId: id,
 				parentSessionPath: options.parentSessionPath ?? sourceMetadata.path,
+				metadata: options.metadata ?? sourceMetadata.metadata,
 			},
 		);
 		for (const entry of forkedEntries) {