Downgrades the "reporting script completed" log in `agentscripts` from
ERROR to WARN.
During agent reconnects, the `scriptCompleted` RPC can race with the
connection teardown, producing a "connection closed" error. Since
`slogtest` treats ERROR logs as test failures, this causes
`TestAgent_ReconnectNoLifecycleReemit` to flake on macOS.
A failed timing report is non-fatal — the script itself has already
finished, and the agent will continue operating normally. WARN is the
appropriate severity, consistent with the call site in
`agent.go:createDevcontainer`.
Also switches from `fmt.Sprintf` to structured `slog.Error` fields for
consistency with the rest of the codebase.
Fixescoder/internal#1410
Spike Curtis
ac51610332