append-run-log

# Append run log ## Contract | Field | Bound contract | |---|---| | Trigger | A completed agent run must be recorded as durable, queryable evidence. | | Authority | Reversible local: writes only the run log by appending; prior entries are never rewritten; rollback is version control. No remote mutation. | | Side effect | Appends one entry to the append-only JSONL run log under an ISO-only date guard, prunes entries older than the declared retention window, and updates the last-run pointer. | | Done | Exactly one entry exists per run with a unique run id, malformed lines are ignored rather than corrupting the log, and aggregate metrics are derivable from the log alone. | ## Inputs - `run_id`: must be supplied, non-empty, and unique within the log. - `started_at` and `ended_at`: must be supplied as ISO 8601 UTC timestamps (`YYYY-MM-DDTHH:mm:ssZ`). The entry date guard is derived from `ended_at`. - `metrics`: must be supplied as a JSON object of run measurements (for example, duration, token count, model, outcome). Aggregates are computed from these fields alone. - `log_path`: must be supplied; path to the append-only JSONL run log. - `last_run_pointer_path`: must be supplied; path to the last-run pointer file. - `retention_window`: optional; an ISO 8601 duration or day count. When omitted, no pruning is performed. ## Procedure 1. Validate inputs at the trust boundary before any mutation: `run_id` is non-empty; `started_at` and `ended_at` parse as ISO 8601 UTC and `ended_a...