Append writes e as a single JSON line to ~/.mooncake/runs.jsonl.
The directory is created if it does not exist. Write failures are
returned but callers should treat them as best-effort.
type Entry struct {
TS time.Time `json:"ts"`
Config string `json:"config"`
Changed int `json:"changed"`
Ok int `json:"ok"`
Skipped int `json:"skipped"`
Failed int `json:"failed"`
DurationMs int64 `json:"duration_ms"`
}